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).

src/nvim/buffer.c

@@ -3631,6 +3631,13 @@ bool bt_prompt(buf_T *buf)
   return buf != NULL && buf->b_p_bt[0] == 'p';
 }
 
+/// @return  true if "buf" is the |cmdwin| scratch buffer.
+bool bt_cmdwin(const buf_T *buf)
+  FUNC_ATTR_PURE
+{
+  return buf != NULL && buf == cmdwin_buf;
+}
+
 /// Open a window for a number of buffers.
 void ex_buffer_all(exarg_T *eap)
 {
@@ -4092,7 +4099,7 @@ char *buf_spname(buf_T *buf)
     if (buf->b_fname != NULL) {
       return buf->b_fname;
     }
-    if (buf == cmdwin_buf) {
+    if (bt_cmdwin(buf)) {
       return _("[Command Line]");
     }
     if (bt_prompt(buf)) {
@@ -4203,34 +4210,6 @@ void wipe_buffer(buf_T *buf, bool aucmd)
   }
 }
 
-/// Creates or switches to a scratch buffer. :h special-buffers
-/// Scratch buffer is:
-///   - buftype=nofile bufhidden=hide noswapfile
-///   - Always considered 'nomodified'
-///
-/// @param bufnr     Buffer to switch to, or 0 to create a new buffer.
-/// @param bufname   Buffer name, or NULL.
-///
-/// @see curbufIsChanged()
-///
-/// @return  FAIL for failure, OK otherwise
-int buf_open_scratch(handle_T bufnr, char *bufname)
-{
-  if (do_ecmd((int)bufnr, NULL, NULL, NULL, ECMD_ONE, ECMD_HIDE, NULL) == FAIL) {
-    return FAIL;
-  }
-  if (bufname != NULL) {
-    apply_autocmds(EVENT_BUFFILEPRE, NULL, NULL, false, curbuf);
-    setfname(curbuf, bufname, NULL, true);
-    apply_autocmds(EVENT_BUFFILEPOST, NULL, NULL, false, curbuf);
-  }
-  set_option_value_give_err(kOptBufhidden, STATIC_CSTR_AS_OPTVAL("hide"), OPT_LOCAL);
-  set_option_value_give_err(kOptBuftype, STATIC_CSTR_AS_OPTVAL("nofile"), OPT_LOCAL);
-  set_option_value_give_err(kOptSwapfile, BOOLEAN_OPTVAL(false), OPT_LOCAL);
-  RESET_BINDING(curwin);
-  return OK;
-}
-
 bool buf_is_empty(buf_T *buf)
 {
   return buf->b_ml.ml_line_count == 1 && *ml_get_buf(buf, 1) == NUL;