An implication of this current approach is that `NVIM_API_LEVEL` should be
bumped when a new Lua function is added.

TODO(future): add a lint check which requires `@since` on all new functions.

ref #25416

- Tags are now created with `[tag]()`
- References are now created with `[tag]`
- Code spans are no longer wrapped

- Added `@inlinedoc` so single use Lua types can be inlined into the
  functions docs. E.g.

  ```lua
  --- @class myopts
  --- @inlinedoc
  ---
  --- Documentation for some field
  --- @field somefield integer

  --- @param opts myOpts
  function foo(opts)
  end
  ```

  Will be rendered as

  ```
  foo(opts)

    Parameters:
      - {opts} (table) Object with the fields:
               - somefield (integer) Documentation
                 for some field
  ```

- Marked many classes with with `@nodoc` or `(private)`.
  We can eventually introduce these when we want to.

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).