docs: auto-generate docs for vim.lpeg and vim.re

- 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`.

runtime/lua/vim/re.lua

@@ -3,6 +3,7 @@
 -- written by Roberto Ierusalimschy
 --
 --- vendored from lpeg-1.1.0
+--- documentation available at runtime/lua/vim/_meta/re.lua
 
 -- imported functions and modules
 local tonumber, type, print, error = tonumber, type, print, error