RegExpUnicodeSets

SerializationFeature.RegExpUnicodeSets

Enable writing RegExp with the UnicodeSets flag.

This wasn’t a format change in the serializer itself, but versions of V8 without support for this flag will not be able to deserialize containing a RegExp using the flag.

The commit adding the v flag was made on 2022-09-03

Note that it seems the flag wasn’t correctly validated by the serializer, so initially V8 could deserialize RegExps that incorrectly used u and v flags at the same time. v8serialize consistently enforces the mutual-exclusion of u and v flags in JSRegExp.