refactor(exmode): Ex-mode as cmdwin + Lua #40991

Problem:
POSIX-compatible Ex-mode requires special-cases all over the codebase to
match various quirks that don't actually matter to users.
- The main utility of *interactive* Ex-mode is its REPL behavior, and
  that can be achieved with `cmdwin`, which also gains extra UX
  benefits.
- The main utility of *non-interactive* `nvim -es` is for shell
  scripting, where Ex-mode quirks are mostly unhelpful (e.g. the
  "Entering Ex mode" message).

Solution:
- Reimplement *interactive* Ex-mode as a "persistent, insert-mode
  cmdwin" in Lua.
  - "nvim -e/-E" is simply an alias to "gQ".
- Reframe *non-interactive* Ex-mode (`nvim -es`) as "script mode".
  - Drop POSIX Ex-mode quirks.

Improvements:
- "nvim -V1 -es" output ends with a final newline!
- "nvim -V1 -es" no longer shows the "Entering Ex mode" msg. (This was
  pointless noise, unwanted for scripting purposes.)
- stdin is no longer typeahead. Scripts (":lua io.read()") can read
  stdin as data.
- Empty line is a no-op: a stray blank line no longer moves the cursor
  (deviates from POSIX ex "+1"), no longer exits 1 at EOF (E501).

Preserved behavior:
- cursor starts at "$"
- mode()=="cv" (for non-interactive)
- multiline commands (:append/:function/heredoc pull continuation lines)
- bare-range print
- :print=>stdout
- -V1=>stderr
- CRLF input
- continue-after-error and exit codes

Dropped (regressed) POSIX behavior (non-interactive):
- Event loop only ticks while/between commands, not while blocked
  waiting for a stdin line.
- ":g/pat/visual...Q"
- input()/getchar()/":s/x/y/c" no longer consume stdin lines as
  answers: Nvim stops at end-of-input, skipping the rest of the script,
  exit 0. Use ":lua io.read()" instead.
  - If users care about this they should use interactive Ex-mode (`gQ`).
- ":@r" stops at end of the register instead of continuing to read
  cmdline input from stdin.

test/functional/api/vim_spec.lua

@@ -5317,30 +5317,16 @@ describe('API', function()
       )
     end)
     it('does not interfere with printing line in Ex mode #19400', function()
-      local screen = Screen.new(60, 7)
+      local screen = Screen.new(60, 12)
       insert([[
         foo
         bar]])
       feed('gQ1')
-      screen:expect([[
-        foo                                                         |
-        bar                                                         |
-        {1:~                                                           }|*2
-        {3:                                                            }|
-        Entering Ex mode.  Type "visual" to go to Normal mode.      |
-        :1^                                                          |
-      ]])
+      screen:expect({ any = vim.pesc('{1::}1^') })
       eq('Parsing command-line', pcall_err(api.nvim_parse_cmd, '', {}))
+      -- Executing the line still auto-prints it.
       feed('<CR>')
-      screen:expect([[
-        foo                                                         |
-        bar                                                         |
-        {3:                                                            }|
-        Entering Ex mode.  Type "visual" to go to Normal mode.      |
-        :1                                                          |
-        foo                                                         |
-        :^                                                           |
-      ]])
+      screen:expect({ any = vim.pesc('" foo') })
     end)
     it('does not move cursor or change search history/pattern #19878 #19890', function()
       api.nvim_buf_set_lines(0, 0, -1, true, { 'foo', 'bar', 'foo', 'bar' })