Set dev_xx.txt help files to use "flow" layout.

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).

- Add section `VIM.LPEG` and `VIM.RE` to docs/lua.txt.

- Add `_meta/re.lua` which adds luadoc and type annotations, for the
  vendored `vim.re` package.

- Fix minor style issues on `_meta/lpeg.lua` luadoc for better vimdocs
  generation.

- Fix a bug on `gen_vimdoc` where non-helptags in verbatim code blocks
  were parsed as helptags, affecting code examples on `vim.lpeg.Cf`,
  etc.

- Also move the `vim.regex` section below so that it can be located
  closer to `vim.lpeg` and `vim.re`.