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/highlight.lua

@@ -1,4 +1,4 @@
----@defgroup vim.highlight
+---@brief
 ---
 --- Nvim includes a function for highlighting a selection on yank.
 ---
@@ -19,18 +19,19 @@
 --- ```vim
 --- au TextYankPost * silent! lua vim.highlight.on_yank {on_visual=false}
 --- ```
+---
 
 local api = vim.api
 
 local M = {}
 
 --- Table with default priorities used for highlighting:
----     - `syntax`: `50`, used for standard syntax highlighting
----     - `treesitter`: `100`, used for treesitter-based highlighting
----     - `semantic_tokens`: `125`, used for LSP semantic token highlighting
----     - `diagnostics`: `150`, used for code analysis such as diagnostics
----     - `user`: `200`, used for user-triggered highlights such as LSP document
----       symbols or `on_yank` autocommands
+--- - `syntax`: `50`, used for standard syntax highlighting
+--- - `treesitter`: `100`, used for treesitter-based highlighting
+--- - `semantic_tokens`: `125`, used for LSP semantic token highlighting
+--- - `diagnostics`: `150`, used for code analysis such as diagnostics
+--- - `user`: `200`, used for user-triggered highlights such as LSP document
+---   symbols or `on_yank` autocommands
 M.priorities = {
   syntax = 50,
   treesitter = 100,