- Added 'spell' option to extmarks:

  Extmarks with this set will have the region spellchecked.

- Added 'noplainbuffer' option to 'spelloptions':

  This is used to tell Neovim not to spellcheck the buffer. The old
  behaviour was to spell check the whole buffer unless :syntax was set.

- Added spelling support to the treesitter highlighter:

  @spell captures in highlights.scm are used to define regions which
  should be spell checked.

- Added support for navigating spell errors for extmarks:

  Works for both ephemeral and static extmarks

- Added '_on_spell_nav' callback for decoration providers:

  Since ephemeral callbacks are only drawn for the visible screen,
  providers must implement this callback to instruct Neovim which
  regions in the buffer need can be spell checked.

  The callback takes a start position and an end position.

  Note: this callback is subject to change hence the _ prefix.

- Added spell captures for built-in support languages

Co-authored-by: Lewis Russell <lewis6991@gmail.com>
Co-authored-by: Björn Linse <bjorn.linse@gmail.com>

This is accomplished with adding a few new parameters to th e float
config:

  * title: The title as a string
  * title_pos: The position of the title
      is one of:
        * kTitleLeft
        * kTitleCenter
        * kTitleRight

The title does support statusline-esque highlighting semantics.

- reimplement 'winhl' in terms of highlight namespaces
- check for EOF in screen tests (to indicate a likely crash)

Ref: https://github.com/vim/vim/commit/84f546363068e4ddfe14a8a2a2322bb8d3a25417
Rename:
- `underlineline` to `underdouble`
- `underdot` to `underdotted`
- `underdash` to `underdashed`

`underdouble` also now takes higher precedence than `undercurl`.

Also fixes a memory leak in `parse_cmdline`.

Closes #18954.