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`.

src/nvim/CMakeLists.txt

@@ -912,6 +912,7 @@ file(GLOB API_SOURCES CONFIGURE_DEPENDS ${PROJECT_SOURCE_DIR}/src/nvim/api/*.c)
 
 file(GLOB LUA_SOURCES CONFIGURE_DEPENDS
   ${NVIM_RUNTIME_DIR}/lua/vim/*.lua
+  ${NVIM_RUNTIME_DIR}/lua/vim/_meta/*.lua
   ${NVIM_RUNTIME_DIR}/lua/vim/filetype/*.lua
   ${NVIM_RUNTIME_DIR}/lua/vim/lsp/*.lua
   ${NVIM_RUNTIME_DIR}/lua/vim/treesitter/*.lua