Integrate with `lex` and `lex-parse-re`

[joddie]

GNU ELPA now has a neat-looking lex library for generating lexers, written by Stefan Monnier, which includes a function lex-parse-re for parsing Emacs and basic regex syntax into rx form much like rxt-elisp-to-rx does.

It would be good to avoid duplicating this functionality, and possibly also rewrite pcre2el's parsers in the same style as lex-parse-re so that they can interoperate.