Use PCRE2 JIT
In #126 we talked about using PCRE2 and it's JIT to improve runtime performance at the cost of some memory (once a language spec is actually used) along with some regex compile time overhead (to generate the JIT).
We can do this on master now that we're breaking ABI with 4.x.
This doesn't exactly break ABI since it's a private implementation detail, but it does get exposed into the language specs in case there are any incompatibilities between GRegex (PCRE based) and PCRE2. Currently I don't see/know of any incompatibilities.