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).

Problem: Any preceding luadocs block that define alias types with
`@alias` magic would be prepended to the documentation of functions
that follow, despite the "blank line" separator. For example:

```
--- @alias some.type.between.functions
--- Blah blah long documentation for alias
--- | "foo" # foo
--- | "bar" # bar

--- The documentation that should appear in vimdoc.
function M.function_to_include_in_doc()
  ...
end
```

then the vimdoc generated for `function_to_include_in_doc` would include
the text from the alias block (e.g., "Blah blah ... for alias").

Solution:

- refactor: Lua2DoxFilter should maintain its own internal state
  `generics`, rather than carrying it as a parameter to local helper
  functions.

- Add another boolean state `boolean_state` which represents whether
  to ignore the current docstring block (magic lines). This flag will
  be reset as soon as the block is end.

Note: As expected, there is no change at all in the current docs
generated, because we have been working around and writing luadoc
comments so that such erroneous docstring resulting from preceding
`@alias` blocks won't appear.

Added a lpeg grammar for LuaCATS and use it in lua2dox.lua

Problem:
We don't enable stylua for many Lua scripts. Automating code-style is an
important tool for reducing time spent on accidental (non-essential)
complexity.

Solution:
- Enable lintlua for `scripts/` directory.
- Specify `call_parentheses = "Input"`, we should allow kwargs-style
  function invocations.

Memoizes a function, using a custom function to hash the arguments.

Private for now until:

- There are other places in the codebase that could benefit from this
  (e.g. LSP), but might require other changes to accommodate.
- Invalidation of the cache needs to be controllable. Using weak tables
  is an acceptable invalidation policy, but it shouldn't be the only
  one.
- I don't think the story around `hash_fn` is completely thought out. We
  may be able to have a good default hash_fn by hashing each argument,
  so basically a better 'concat'.

Everything after a "#" char is a "description" comment, i.e. luals won't
treat it as a type, name, etc. But "#" should not be present in the
generated docs (such as :help docs).
https://github.com/LuaLS/lua-language-server/wiki/Annotations#return

* docs(lua): teach lua2dox how to table

* docs(lua): teach gen_vimdoc.py about local functions

No more need to mark local functions with @private

* docs(lua): mention @nodoc and @meta in dev-lua-doc

* fixup!

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

---------

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>