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`.

MAINTAIN.md

@@ -156,8 +156,11 @@ These dependencies are "vendored" (inlined), we must update the sources manually
     * Run `scripts/gen_lsp.lua` to update.
 * `runtime/lua/vim/_meta/lpeg.lua`: LPeg definitions.
     * Refer to [`LuaCATS/lpeg`](https://github.com/LuaCATS/lpeg) for updates.
+    * Update the git SHA revision from which the documentation was taken.
 * `runtime/lua/vim/re.lua`: LPeg regex module.
     * Vendored from LPeg. Needs to be updated when LPeg is updated.
+* `runtime/lua/vim/_meta/re.lua`: docs for LPeg regex module.
+    * Needs to be updated when LPeg is updated.
 * `src/bit.c`: only for PUC lua: port of `require'bit'` from luajit https://bitop.luajit.org/
 * `runtime/lua/coxpcall.lua`: coxpcall (only needed for PUC lua, builtin to luajit)
 * `src/termkey`: [libtermkey](https://github.com/neovim/libtermkey)