Get js-regex adopted by jsonschema and hypothesis-jsonschema

[Zac-HD]

There's not much point in building this if it's not used!

For reference, this package was inspired by python-jsonschema/hypothesis-jsonschema#22.

[lsorber]

Nice, didn't realise you had already started working on this. Looking forward to the benefits it will bring!

[Zac-HD]

Closing this issue because the upstream schema spec is incoherent with respect to regex treatment, and of the allowed options "use native" regex seems like the recommendation (despite being against my original reading of the spec).