feat(options): schema, "dict" options, messages

Problem:
Options parsing is still painful for dict-style options.

Solution:
schema-maxxing => better `opt:get()` (will be the basis for `vim.o()`),
unified (and more-detailed) err msgs.

- Drop bespoke structure-builder in `_core/options.lua`.
- Define `schema` for all non-primitive options (except 'guicursor' and
  statusline-style options); generate reified keysets `OptKeyDict`).
  - Generate 'fillchars' => `fcs_tab`, 'listchars' => `lcs_tab`.
- `nvim_set_option_value`:
  - Return the improved structures. Also from `vim.opt.x:get()`.
  - Eliminate api <=> lua roundtrip, centralize option structure
    handling.
- Improve/unify errors.
  - Bump ERR_BUFLEN 80 → 256 so the "one of" list isn't truncated.
- Eliminate old 'diffopt' order-dependence (`iwhiteall` before `iwhite`)

Error samples:

    Typed-key path (opt_strings_check → diffopt/mousescroll/breakindentopt):
    E474: Unknown item 'foo'
    E474: 'context' requires a number
    E474: 'ver' number is out of range
    E474: 'algorithm' must be one of: myers, minimal, patience, histogram
    E474: 'filler' does not take a value

Related:

- #31084
- #34661
- #31820
- #14739
- #20107
- fix #18875
  - :get() returns `{ sbr = true, shift = '3' }` (reified-keyset) instead of `{'sbr', 'shift:3'}`
  - Setting via table now works too. `object_as_optval_for` `is_map` now recognizes struct options.
- fix #30296
  - instead of `E474: Invalid argument`, errors now look like:
    ```
    E474: Invalid value 'x', expected one of: single, double: ambiwidth=x
    E474: Unknown item 'foo': diffopt=foo
    E474: 'context' requires a number: diffopt=context:x
    ```

simplify `win_float_parse_option` from #26799.

src/nlua0.zig

@@ -13,6 +13,7 @@ const embedded_data = @import("embedded_data");
 
 // these are common dependencies used by many generators
 const hashy = @embedFile("gen/hashy.lua");
+const keyset = @embedFile("gen/keyset.lua");
 const c_grammar = @embedFile("gen/c_grammar.lua");
 
 const Lua = ziglua.Lua;
@@ -43,6 +44,8 @@ fn init_lua() !*Lua {
     _ = lua.getField(-1, "preload");
     try lua.loadBuffer(hashy, "hashy.lua"); // [package, preload, hashy]
     lua.setField(-2, "gen.hashy");
+    try lua.loadBuffer(keyset, "keyset.lua"); // [package, preload, keyset]
+    lua.setField(-2, "gen.keyset");
     try lua.loadBuffer(c_grammar, "c_grammar.lua"); // [package, preload, c_grammar]
     lua.setField(-2, "gen.c_grammar");
     lua.pop(2);