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/vimscript.c

@@ -351,9 +351,7 @@ typedef struct {
   Object *ret_node_p;
 } ExprASTConvStackItem;
 
-/// @cond DOXYGEN_NOT_A_FUNCTION
 typedef kvec_withinit_t(ExprASTConvStackItem, 16) ExprASTConvStack;
-/// @endcond
 
 /// Parse a Vimscript expression.
 ///
@@ -387,8 +385,8 @@ typedef kvec_withinit_t(ExprASTConvStackItem, 16) ExprASTConvStack;
 ///          - "arg": String, error message argument.
 ///        - "len": Amount of bytes successfully parsed. With flags equal to ""
 ///                 that should be equal to the length of expr string.
-///                 (“Successfully parsed” here means “participated in AST
-///                  creation”, not “till the first error”.)
+///                 ("Successfully parsed" here means "participated in AST
+///                  creation", not "till the first error".)
 ///        - "ast": AST, either nil or a dictionary with these keys:
 ///          - "type": node type, one of the value names from ExprASTNodeType
 ///                    stringified without "kExprNode" prefix.