Breaking: Throw for [^\p{…}] with flag i in environments without native flag v

[slevithan]

regex v2.1.0 introduced src/backcompat.js, which extended support for most usage backward to environments without native flag v. This file is a postprocessor that is conditionally applied when flag v is not supported natively, and it does several things:

1. Transpiles flag v's escaping rules to u's (by un-escaping some characters).
2. Throws for character class syntax that is invalid with v but valid with u (unescaped (){}/|, reserved double punctuators, and leading/trailing -, since all of these would throw in environments with v).
3. Throws for features that require native flag v (character class set operators -- and &&, and a more descriptive error for nested character classes which are already invalid with u).
4. Throws when using doubly-negated [^\P{…}] if the regex uses flag i, to prevent an unintuitive (likely) behavior difference (likely, since many but not all Unicode properties include letters with case).

Regarding # 4, this is fine, but it's incomplete. At the time, I was referencing this and this, which only show examples with doubly-negated [^\P{…}]. However, as @rauschma points out here, this v/u incompatibility applies equally with \p (lowercase) in /[^\p{…}]/iu.

Updating this in src/backcompat.js to also cover \p would be easy, but would also be a (rarely applicable) breaking change. IMO it wouldn't be a problem to start throwing without a new major version if the behavior was always incompatible (since anyone with such patterns in their code would already have a bug in environments without v), but since not all uses of /[^\p{…}]/iu suffer from this issue, I will hold off on updating it until there are additional breaking changes to include in a v4.0.

[slevithan]

Rather than throwing for [^\p{…}] with flag i (when v is not supported natively), I resolved this by:

• Not throwing for uppercase [^\P{…}] with flag i, either (since it's not actually always problematic).
• Introducing new option unicodeSetsPlugin, which allows overriding the built-it plugin for backcompat with a more robust transpiler (e.g. based on regexpu-core), if needed.