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/version.lua

@@ -1,6 +1,5 @@
---- @defgroup vim.version
----
---- @brief The \`vim.version\` module provides functions for comparing versions and ranges
+--- @brief
+--- The `vim.version` module provides functions for comparing versions and ranges
 --- conforming to the https://semver.org spec. Plugins, and plugin managers, can use this to check
 --- available tools and dependencies on the current system.
 ---
@@ -13,9 +12,9 @@
 --- end
 --- ```
 ---
---- \*vim.version()\* returns the version of the current Nvim process.
+--- *vim.version()* returns the version of the current Nvim process.
 ---
---- VERSION RANGE SPEC \*version-range\*
+--- VERSION RANGE SPEC *version-range*
 ---
 --- A version "range spec" defines a semantic version range which can be tested against a version,
 --- using |vim.version.range()|.