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).

src/nvim/api/command.c

@@ -67,7 +67,7 @@
 ///             - file: (boolean) The command expands filenames. Which means characters such as "%",
 ///                               "#" and wildcards are expanded.
 ///             - bar: (boolean) The "|" character is treated as a command separator and the double
-///                              quote character (\") is treated as the start of a comment.
+///                              quote character (") is treated as the start of a comment.
 ///         - mods: (dictionary) |:command-modifiers|.
 ///             - filter: (dictionary) |:filter|.
 ///                 - pattern: (string) Filter pattern. Empty string if there is no filter.