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).

contrib/flake.nix

@@ -140,7 +140,6 @@
                 include-what-you-use # for scripts/check-includes.py
                 jq # jq for scripts/vim-patch.sh -r
                 shellcheck # for `make shlint`
-                doxygen # for script/gen_vimdoc.py
               ];
 
             nativeBuildInputs = with pkgs;

contrib/local.mk.example

@@ -55,9 +55,3 @@
 # DEPS_CMAKE_FLAGS += -DUSE_BUNDLED=OFF
 
 # .DEFAULT_GOAL := nvim
-#
-# Run doxygen over the source code.
-# Output will be in build/doxygen
-#
-# doxygen:
-# 	doxygen src/Doxyfile