feat(docs): replace lua2dox.lua

Problem:

The documentation flow (`gen_vimdoc.py`) has several issues:
- it's not very versatile
- depends on doxygen
- doesn't work well with Lua code as it requires an awkward filter script to convert it into pseudo-C.
- The intermediate XML files and filters makes it too much like a rube goldberg machine.

Solution:

Re-implement the flow using Lua, LPEG and treesitter.

- `gen_vimdoc.py` is now replaced with `gen_vimdoc.lua` and replicates a portion of the logic.
- `lua2dox.lua` is gone!
- No more XML files.
- Doxygen is now longer used and instead we now use:
  - LPEG for comment parsing (see `scripts/luacats_grammar.lua` and `scripts/cdoc_grammar.lua`).
  - LPEG for C parsing (see `scripts/cdoc_parser.lua`)
  - Lua patterns for Lua parsing (see `scripts/luacats_parser.lua`).
  - Treesitter for Markdown parsing (see `scripts/text_utils.lua`).
- The generated `runtime/doc/*.mpack` files have been removed.
   - `scripts/gen_eval_files.lua` now instead uses `scripts/cdoc_parser.lua` directly.
- Text wrapping is implemented in `scripts/text_utils.lua` and appears to produce more consistent results (the main contributer to the diff of this change).

runtime/lua/vim/_meta/re.lua

@@ -7,15 +7,12 @@ error('Cannot require a meta file')
 -- Copyright © 2007-2023 Lua.org, PUC-Rio.
 -- See 'lpeg.html' for license
 
---- @defgroup vim.re
----<pre>help
----The `vim.re` module provides a conventional regex-like syntax for pattern usage
----within LPeg |vim.lpeg|.
+--- @brief
+--- The `vim.re` module provides a conventional regex-like syntax for pattern usage
+--- within LPeg |vim.lpeg|.
 ---
----See https://www.inf.puc-rio.br/~roberto/lpeg/re.html for the original
----documentation including regex syntax and more concrete examples.
----
----</pre>
+--- See https://www.inf.puc-rio.br/~roberto/lpeg/re.html for the original
+--- documentation including regex syntax and more concrete examples.
 
 --- Compiles the given {string} and returns an equivalent LPeg pattern. The given string may define
 --- either an expression or a grammar. The optional {defs} table provides extra Lua values to be used