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/ui.c

@@ -165,7 +165,7 @@ void remote_ui_wait_for_attach(bool only_stdio)
 
 /// Activates UI events on the channel.
 ///
-/// Entry point of all UI clients.  Allows |\-\-embed| to continue startup.
+/// Entry point of all UI clients.  Allows |--embed| to continue startup.
 /// Implies that the client is ready to show the UI.  Adds the client to the
 /// list of UIs. |nvim_list_uis()|
 ///
@@ -541,7 +541,7 @@ void nvim_ui_pum_set_bounds(uint64_t channel_id, Float width, Float height, Floa
 ///
 /// @param channel_id
 /// @param event Event name
-/// @param payload Event payload
+/// @param value Event payload
 /// @param[out] err Error details, if any.
 void nvim_ui_term_event(uint64_t channel_id, String event, Object value, Error *err)
   FUNC_API_SINCE(12) FUNC_API_REMOTE_ONLY