feat(ui): ext_cmdline/messages for the TUI #27855

Problem:  We have an unmaintained Vimscript parser and cmdline
highlighting mechanism, with which it is hard to leverage the
treesitter highlighter. Long messages result in a hit-enter-prompt.

Solution: Implement a vim.ui_attach() UI, that replaces the message
grid (orphaning some 3000+ LOC core C code). Introduce an experimental
vim._extui module, because removing the message grid at the same time is
too risky. The new UI leverages the bundled treesitter highlighter and
parser for Vimscript, as well as the matchparen plugin, to highlight the
cmdline. Messages are truncated in the cmdline area, or placed in a
floating message box in the bottom right corner. Special ("list_cmd")
messages and the message history are shown in a, "more prompt" (now a
fully interactive regular window). Various default UI elements ('showcmd',
'ruler') are also placed in the cmdline area, as virtual text.

`require('vim._extui').enable({})` enables the experimental UI.
`{ msg.pos = 'box' }` or `:set cmdheight=0` enables the message
box variant.

Followup:
  - Come to a consensus for how best to represent messages (by default).
  - Start removing message grid when this is deemed a successful replacement.
    When that is finished, make this new UI the default and update a lot of tests.

src/gen/gen_vimdoc.lua

@@ -149,6 +149,7 @@ local config = {
       'loader.lua',
       'uri.lua',
       'ui.lua',
+      '_extui.lua',
       'filetype.lua',
       'keymap.lua',
       'fs.lua',
@@ -171,6 +172,7 @@ local config = {
       'runtime/lua/vim/loader.lua',
       'runtime/lua/vim/uri.lua',
       'runtime/lua/vim/ui.lua',
+      'runtime/lua/vim/_extui.lua',
       'runtime/lua/vim/filetype.lua',
       'runtime/lua/vim/keymap.lua',
       'runtime/lua/vim/fs.lua',