feat(options)!: disallow setting hidden options #28400

Problem:
There are three different ways of marking an option as hidden, `enable_if
= false`, `hidden = true` and `immutable = true`. These also have different
behaviors. Options hidden with `enable_if = false` can't have their value
fetched using Vim script or the API, but options hidden with `hidden = true` or
`immutable = true` can. On the other hand, options with `hidden = true` do not
error when trying to set their value, but options with `immutable = true` do.

Solution:
Remove `enable_if = false`, remove the `hidden` property for options, and use
`immutable = true` to mark an option as hidden instead. Also make hidden option
variable pointers always point to the default value, which allows fetching the
value of every hidden option using Vim script and the API. This does also mean
that trying to set a hidden option will now give an error instead of just being
ignored.

src/nvim/options.lua

@@ -13,7 +13,7 @@
 --- @field list? 'comma'|'onecomma'|'commacolon'|'onecommacolon'|'flags'|'flagscomma'
 --- @field scope vim.option_scope[]
 --- @field deny_duplicates? boolean
---- @field enable_if? string|false
+--- @field enable_if? string
 --- @field defaults? vim.option_defaults
 --- @field secure? true
 --- @field noglob? true
@@ -87,11 +87,11 @@ return {
     {
       abbreviation = 'al',
       defaults = { if_true = 224 },
-      enable_if = false,
       full_name = 'aleph',
       scope = { 'global' },
       short_desc = N_('ASCII code of the letter Aleph (Hebrew)'),
       type = 'number',
+      immutable = true,
     },
     {
       abbreviation = 'ari',
@@ -789,11 +789,11 @@ return {
            current	Use the current directory.
            {path}	Use the specified directory
       ]=],
-      enable_if = false,
       full_name = 'browsedir',
       scope = { 'global' },
       short_desc = N_('which directory to start browsing in'),
       type = 'string',
+      immutable = true,
     },
     {
       abbreviation = 'bh',
@@ -1493,7 +1493,7 @@ return {
       cb = 'did_set_completeslash',
       defaults = { if_true = '' },
       desc = [=[
-        		only for MS-Windows
+        		only modifiable in MS-Windows
         When this option is set it overrules 'shellslash' for completion:
         - When this option is set to "slash", a forward slash is used for path
           completion in insert mode. This is useful when editing HTML tag, or
@@ -3791,12 +3791,12 @@ return {
         	try to keep 'lines' and 'columns' the same when adding and
         	removing GUI components.
       ]=],
-      enable_if = false,
       full_name = 'guioptions',
       list = 'flags',
       scope = { 'global' },
       short_desc = N_('GUI: Which components and options are used'),
       type = 'string',
+      immutable = true,
     },
     {
       abbreviation = 'gtl',
@@ -3816,13 +3816,13 @@ return {
         present in 'guioptions'.  For the non-GUI tab pages line 'tabline' is
         used.
       ]=],
-      enable_if = false,
       full_name = 'guitablabel',
       modelineexpr = true,
       redraw = { 'current_window' },
       scope = { 'global' },
       short_desc = N_('GUI: custom label for a tab page'),
       type = 'string',
+      immutable = true,
     },
     {
       abbreviation = 'gtt',
@@ -3835,12 +3835,12 @@ return {
         	let &guitabtooltip = "line one\nline two"
         <
       ]=],
-      enable_if = false,
       full_name = 'guitabtooltip',
       redraw = { 'current_window' },
       scope = { 'global' },
       short_desc = N_('GUI: custom tooltip for a tab page'),
       type = 'string',
+      immutable = true,
     },
     {
       abbreviation = 'hf',
@@ -4082,11 +4082,11 @@ return {
         English characters directly, e.g., when it's used to type accented
         characters with dead keys.
       ]=],
-      enable_if = false,
       full_name = 'imcmdline',
       scope = { 'global' },
       short_desc = N_('use IM when starting to edit a command line'),
       type = 'boolean',
+      immutable = true,
     },
     {
       abbreviation = 'imd',
@@ -4100,11 +4100,11 @@ return {
         Currently this option is on by default for SGI/IRIX machines.  This
         may change in later releases.
       ]=],
-      enable_if = false,
       full_name = 'imdisable',
       scope = { 'global' },
       short_desc = N_('do not use the IM in any mode'),
       type = 'boolean',
+      immutable = true,
     },
     {
       abbreviation = 'imi',
@@ -5236,7 +5236,7 @@ return {
       scope = { 'global' },
       short_desc = N_('maximum nr of combining characters displayed'),
       type = 'number',
-      hidden = true,
+      varname = 'p_mco',
     },
     {
       abbreviation = 'mfd',
@@ -5732,13 +5732,13 @@ return {
         indicate no input when the hit-enter prompt is displayed (since
         clicking the mouse has no effect in this state.)
       ]=],
-      enable_if = false,
       full_name = 'mouseshape',
       list = 'onecomma',
       scope = { 'global' },
       short_desc = N_('shape of the mouse pointer in different modes'),
       tags = { 'E547' },
       type = 'string',
+      immutable = true,
     },
     {
       abbreviation = 'mouset',
@@ -5898,11 +5898,11 @@ return {
         Note that on Windows editing "aux.h", "lpt1.txt" and the like also
         result in editing a device.
       ]=],
-      enable_if = false,
       full_name = 'opendevice',
       scope = { 'global' },
       short_desc = N_('allow reading/writing devices on MS-Windows'),
       type = 'boolean',
+      immutable = true,
     },
     {
       abbreviation = 'opfunc',
@@ -5975,11 +5975,11 @@ return {
     {
       abbreviation = 'pt',
       defaults = { if_true = '' },
-      enable_if = false,
       full_name = 'pastetoggle',
       scope = { 'global' },
       short_desc = N_('No description'),
       type = 'string',
+      immutable = true,
     },
     {
       abbreviation = 'pex',
@@ -7270,9 +7270,14 @@ return {
     {
       abbreviation = 'ssl',
       cb = 'did_set_shellslash',
-      defaults = { if_true = false },
+      defaults = {
+        condition = 'MSWIN',
+        if_true = false,
+        if_false = true,
+        doc = 'on, Windows: off',
+      },
       desc = [=[
-        		only for MS-Windows
+        		only modifiable in MS-Windows
         When set, a forward slash is used when expanding file names.  This is
         useful when a Unix-like shell is used instead of cmd.exe.  Backward
         slashes can still be typed, but they are changed to forward slashes by
@@ -8885,11 +8890,11 @@ return {
     {
       abbreviation = 'tenc',
       defaults = { if_true = '' },
-      enable_if = false,
       full_name = 'termencoding',
       scope = { 'global' },
       short_desc = N_('Terminal encoding'),
       type = 'string',
+      immutable = true,
     },
     {
       abbreviation = 'tgc',