Remove $unsupported tokens in the regex lexers

[katef]

Remove $unsupported tokens in the regex lexers

[sfstewman]

I think this makes a lot of sense.

For the PCRE dialect, $unsupported currently falls into four buckets:

• Word boundary, capture groups, and multiline things that libfsm could potentially support: \b, \B, \K, \Z, and \G.
• Back references. It makes sense to include these in the AST; simple forms like (foo)\1 can be transformed into (foo)(?:foo) which is compatible with DFAs and linear scanning.
• Positive/negative look-ahead and look-behind assertions. We may be able to transform these into something compatible with linear scanning.
• Ways to control backtracking: atomic qualifiers and (*VERB) forms like (*COMMIT) and (*PRUNE). These are so specific to backtracking matchers (and PCRE) that I'm not sure if we want to include them. On the other hand, there aren't that many forms of this, so it may make sense.