Support absence groups

[slevithan]

Oniguruma supports multiple forms of "absence" operators/functions/groups. The basic form (?~…) is extremely rarely used, but at least has good use cases. Other forms (that start with (?~|) are so exceedingly rare that they're probably not worth supporting (and some are likely not emulatable anyway).

On rarity: The basic form was used by two regexes out of tens of thousands in a sample of real-world Oniguruma regexes used in TextMate grammars. The other forms were not used at all.

If I understand Oniguruma's basic form (?~…) correctly, it can be emulated in JS as (?:(?!…)\p{Any})*. Running a few basic tests in Oniguruma show this to be producing the same results. This is a trivial transformation to do in src/transform.js after first adding support for parsing absence operators in src/tokenize.js and src/parse.js.

Additionally:

• Absence operators currently throw a custom error here.
• The behavior of (?~…) is different in Oniguruma and Onigmo. So Ruby regex testers like rubular.com are not helpful.
• Need to test the effects of quantification, whether absence operators are atomic, etc.
• Should catch non-basic forms that aren't supported (starting with (?~|) as an error.
  • It might be easy to also support the "absent expression" form (?~|absent|exp).
• Oniguruma itself doesn't support "absent stopper" and "range clear" forms within lookbehind.