Problem:
`AucmdExecutable` adds 2 layers of indirection. Although formalizing
a `union` is good practice for shared interfaces, this struct is mainly
for `autocmd_register` which is internal to this module.

Solution:
- Store the cmd/fn fields directly on the `AutoCmd` struct.
- Drop `AucmdExecutable` and related structures.

Co-authored-by: Dustin S. <dstackmasta27@gmail.com>
Co-authored-by: Ferenc Fejes <fejes@inf.elte.hu>
Co-authored-by: Maria José Solano <majosolano99@gmail.com>
Co-authored-by: Yochem van Rosmalen <git@yochem.nl>
Co-authored-by: brianhuster <phambinhanctb2004@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Problem:
`nvim_win_set_config` cannot set the title and footer independently.
When only one is given, the other is reset to the default of "left".

Solution:
Reuse existing title/footer value if not provided.

Problem:
nvim_get_autocmds cannot filter by id.

Solution:
Support it.

Problem: Right aligned virtual text can cover up buffer text if virtual
text is too long

Solution: An additional option for `virt_text_pos` called
`eol_right_align` has been added to truncate virtual text if it would
have otherwise covered up buffer text. This ensures the virtual text
extends no further left than EOL.

Problem:  Combined highlighting was not applied to nvim_eval_statusline(),
          and 'statuscolumn' sign segment/numhl highlights.
Solution: Add an additional `groups` element to the return value of
          `nvim_eval_statusline()->highlights`. This is an array of stacked
          highlight groups (highest priority last). Also resolve combined
          highlights for the 'statuscolumn' sign segment/numhl highlights.
          Expose/synchronize some drawline.c logic that is now mimicked in
          three different places.

feat(extmark): stack multiple highlight groups in `hl_group`

This has been possible in the "backend" for a while but
API was missing.

Followup: we will need a `details2=true` mode for `nvim_get_hl_id_by_name`
to return information in a way forward compatible with even further
enhancements.

This was kept for a while as it was a useful short hand and initially
matched what highlights what actually properly implemented. But now
|vim.hl.range()| is a better high-level shorthand with full support for
native multi-line ranges.