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/_meta/re.lua

@@ -0,0 +1,57 @@
+--- @meta
+error('Cannot require a meta file')
+
+-- Documentations and Lua types for vim.re (vendored re.lua, lpeg-1.1.0)
+-- https://www.inf.puc-rio.br/~roberto/lpeg/re.html
+--
+-- Copyright © 2007-2023 Lua.org, PUC-Rio.
+-- See 'lpeg.html' for license
+
+--- @defgroup vim.re
+---<pre>help
+---The `vim.re` module provides a conventional regex-like syntax for pattern usage
+---within LPeg |vim.lpeg|.
+---
+---See https://www.inf.puc-rio.br/~roberto/lpeg/re.html for the original
+---documentation including regex syntax and more concrete examples.
+---
+---</pre>
+
+--- Compiles the given {string} and returns an equivalent LPeg pattern. The given string may define
+--- either an expression or a grammar. The optional {defs} table provides extra Lua values to be used
+--- by the pattern.
+--- @param string string
+--- @param defs? table
+--- @return vim.lpeg.Pattern
+function vim.re.compile(string, defs) end
+
+--- Searches the given {pattern} in the given {subject}. If it finds a match, returns the index
+--- where this occurrence starts and the index where it ends. Otherwise, returns nil.
+---
+--- An optional numeric argument {init} makes the search starts at that position in the subject
+--- string. As usual in Lua libraries, a negative value counts from the end.
+--- @param subject string
+--- @param pattern vim.lpeg.Pattern|string
+--- @param init? integer
+--- @return integer|nil the index where the occurrence starts, nil if no match
+--- @return integer|nil the index where the occurrence ends, nil if no match
+function vim.re.find(subject, pattern, init) end
+
+--- Does a global substitution, replacing all occurrences of {pattern} in the given {subject} by
+--- {replacement}.
+--- @param subject string
+--- @param pattern vim.lpeg.Pattern|string
+--- @param replacement string
+--- @return string
+function vim.re.gsub(subject, pattern, replacement) end
+
+--- Matches the given {pattern} against the given {subject}, returning all captures.
+--- @param subject string
+--- @param pattern vim.lpeg.Pattern|string
+--- @param init? integer
+--- @return integer|vim.lpeg.Capture|nil
+--- @see vim.lpeg.match()
+function vim.re.match(subject, pattern, init) end
+
+--- Updates the pre-defined character classes to the current locale.
+function vim.re.updatelocale() end