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/editor/cmdwin_spec.lua

@@ -0,0 +1,115 @@
+-- Tests for non-blocking cmdwin. #40312
+
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local feed = n.feed
+local api = n.api
+local fn = n.fn
+local exec_lua = n.exec_lua
+local eq = t.eq
+
+describe('cmdwin', function()
+  before_each(n.clear)
+
+  it('q: opens cmdwin', function()
+    feed('q:')
+    eq(':', fn.getcmdwintype())
+    eq('[Command Line]', vim.fs.basename(api.nvim_buf_get_name(0)))
+    -- cmdwin-char is shown in window-local 'statuscolumn'.
+    eq('%#NonText#: ', api.nvim_get_option_value('statuscolumn', { win = 0 }))
+
+    eq('nofile', api.nvim_get_option_value('buftype', { buf = 0 }))
+    eq('wipe', api.nvim_get_option_value('bufhidden', { buf = 0 }))
+    eq(false, api.nvim_get_option_value('swapfile', { buf = 0 }))
+    eq(true, api.nvim_get_option_value('winfixbuf', { win = 0 }))
+
+    -- <CR> executes the cmdline
+    feed('ilet g:cmdwin_result = 42<Esc>')
+    feed('<CR>')
+    eq(42, api.nvim_get_var('cmdwin_result'))
+    eq('', fn.getcmdwintype())
+  end)
+
+  it('q/ opens cmdwin with search history', function()
+    feed('q/')
+    eq('/', fn.getcmdwintype())
+  end)
+
+  it('c_CTRL-F opens cmdwin pre-filled with current cmdline', function()
+    feed(':echo "hi"<C-F>')
+    n.poke_eventloop()
+    eq(':', fn.getcmdwintype())
+    eq('echo "hi"', api.nvim_get_current_line())
+  end)
+
+  it('<C-C> in normal mode cancels without executing', function()
+    feed('q:')
+    feed('iecho "nope"<Esc>')
+    feed('<C-C>')
+    eq('', fn.getcmdwintype())
+    -- v:errmsg should not contain a successful echo from the cancelled line.
+  end)
+
+  it('async API calls work while cmdwin is open #40312', function()
+    feed('q:')
+    eq(':', fn.getcmdwintype())
+    local ok, err = pcall(function()
+      local b = api.nvim_create_buf(true, false)
+      api.nvim_buf_delete(b, { force = true })
+    end)
+    eq(true, ok, 'buf_delete unexpectedly failed: ' .. tostring(err))
+  end)
+
+  it('textlock does not include cmdwin #40312', function()
+    feed('q:')
+    -- Setting an option from API used to fail with E11 in cmdwin.
+    api.nvim_set_option_value('wildignore', '*.tmp', {})
+  end)
+
+  it('E1292 cannot nest cmdwin', function()
+    feed('q:')
+    eq(':', fn.getcmdwintype())
+    feed('q:')
+    -- Still just one cmdwin.
+    eq(':', fn.getcmdwintype())
+    local nwin = 0
+    for _, w in ipairs(api.nvim_list_wins()) do
+      if api.nvim_get_option_value('winfixbuf', { win = w }) then
+        nwin = nwin + 1
+      end
+    end
+    eq(1, nwin)
+  end)
+
+  it(':quit closes cmdwin', function()
+    feed('q:')
+    eq(':', fn.getcmdwintype())
+    feed(':q<CR>')
+    eq('', fn.getcmdwintype())
+  end)
+
+  it(':messages from inside cmdwin works #40312', function()
+    -- Before #40312, this raised E11 because text_locked() returned true for cmdwin.
+    exec_lua([[vim.api.nvim_echo({{'hello from history'}}, true, {})]])
+    feed('q:')
+    eq(':', fn.getcmdwintype())
+    n.command('messages') -- Previously E11; now actually runs.
+  end)
+
+  it('CmdwinEnter/CmdwinLeave events', function()
+    exec_lua([[
+      _G.events = {}
+      vim.api.nvim_create_autocmd('CmdwinEnter', {
+        callback = function(a) table.insert(_G.events, 'enter:'..a.match) end,
+      })
+      vim.api.nvim_create_autocmd('CmdwinLeave', {
+        callback = function(a) table.insert(_G.events, 'leave:'..a.match) end,
+      })
+    ]])
+    feed('q:')
+    n.poke_eventloop() -- Ensure q: is processed before the <C-C> mapping fires.
+    feed('<C-C>')
+    eq({ 'enter::', 'leave::' }, exec_lua('return _G.events'))
+  end)
+end)