feat(cmdwin): implement cmdwin as a normal buf+win

Problem:

cmdwin (the `:q` cmdline buffer) has various limitations which require
special-casing all over the codebase.

Besides complicating the code, it also breaks async plugins if they try
to create buffers/windows after some work is done, if the user happens
to open cmdwin at the wrong the moment:

    Lua callback: …/guh.nvim/lua/guh/util.lua:531:
    E11: Invalid in command-line window; <CR> executes, CTRL-C quits
    stack traceback:
        [C]: in function 'nvim_buf_delete'
        …/guh.nvim/lua/guh/util.lua:531: in function <…/guh.nvim/lua/guh/util.lua:526>

Solution:

Just say no to "inception". Reimplement cmdwin as a normal buffer+window.

All of the cmdwin contortions (in both core, and innocent plugins) exist
literally only to support "inception": recursive
cmdwin-in-cmdline-things, like `<c-r>=`, `/`, search-during-substitute,
`:input()`, etc. So we just won't support that (though I have
a potential plan for that later, which I call "modal parking lot").

The benefit is that plugins, and core, no longer have to care about
cmdwin.

BONUS:
- mouse-drag on vertical separators works (it only worked for
  horizontal/statusline before)
- inccommand-in-cmdwin now works correctly, for free (thus don't need
  #40077).

POTENTIAL FOLLOWUPS
- Drop `CHECK_CMDWIN` ("E11: Invalid in command-line window"), allow chaos.
- Unify `BUFLOCK_OK` / `LOCK_OK` ?

DESIGN:
- Eliminate lots of C globals, `EX_CMDWIN`, etc.
- `text_locked()` no longer reports true for cmdwin.
- cmdwin = a normal window with 'winfixbuf', 'bufhidden=wipe',
  'buftype=nofile'. Invariants come from those options rather than
  special cases throughout the codebase.
- `nv_record` for q:/q//q? calls Lua
  `nlua_call_vimfn("vim._core.cmdwin", …)`. No `K_CMDWIN`
  / cmdline-reader detour.
- `cedit_key` (`c_CTRL-F`) schedules a deferred event that calls
  `vim._core.cmdwin.open(type, content, pos)` and returns `Ctrl_C` so
  the in-flight cmdline cancels. Reader state is not serialized; instead
  the captured `(type, line, col)` is replayed via
  `nvim_feedkeys(type..line.."<CR>", "nt", …)` after user confirms.
- On confirm/cancel: `<CR>` / `<C-C>` calls into Lua which closes the
  window and re-feeds the cmdline.

BREAKING CHANGES:
- Expression-register cmdline (`<C-R>=` from insert-mode) no longer
  supports cmdwin. Same applies to `input()` / `inputlist()` (already
  covered by `text_locked`).
- Usage of cmdwin in macros/mappings will probably break (assuming they
  ever worked).

test/functional/vimscript/api_functions_spec.lua

@@ -10,7 +10,6 @@ local insert, pcall_err = n.insert, t.pcall_err
 local matches = t.matches
 local api = n.api
 local feed = n.feed
-local command = n.command
 
 describe('eval-API', function()
   before_each(clear)
@@ -91,16 +90,22 @@ describe('eval-API', function()
       api.nvim_get_vvar('errmsg')
     )
 
-    -- Some functions checking textlock (usually those that may change the current window or buffer)
-    -- also ought to not be usable in the cmdwin.
+    -- cmdwin behavior (changed since #40312).
     local old_win = api.nvim_get_current_win()
     feed('q:')
+    local cmdwin_win = api.nvim_get_current_win()
     eq(
-      'E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
+      'Vim:E11: Invalid in command-line window; <CR> executes, CTRL-C quits',
       pcall_err(api.nvim_set_current_win, old_win)
     )
+    eq(old_win, api.nvim_get_current_win())
+    -- TODO(justinmk): awkward: E11 is raised but the focus switch happens anyway; inherited bug in
+    -- goto_tabpage_win (calls win_enter unconditionally even when goto_tabpage_tp's CHECK_CMDWIN
+    -- emsg'd). We should probably just remove CHECK_CMDWIN from goto_tabpage_tp. #40407
+    pcall(api.nvim_set_current_win, cmdwin_win)
+    eq(cmdwin_win, api.nvim_get_current_win())
 
-    -- But others, like nvim_buf_set_lines(), which just changes text, is OK.
+    -- nvim_buf_set_lines() in the cmdwin buffer is OK.
     api.nvim_buf_set_lines(0, 0, -1, 1, { 'wow!' })
     eq({ 'wow!' }, api.nvim_buf_get_lines(0, 0, -1, 1))