`no-useless-flag` false positive for `i` flag and Unicode property escapes

[RunDevelopment]

See the following example:

var re1 = /\p{Ll}/iu;

This report is incorrect as \p{Ll} matches all lower-case letters. The i flag is very much necessary.

[RunDevelopment]

This is most likely caused by an upstream bug in refa.

I just fixed that Unicode property escapes were incorrectly canonicalized. This was incorrect because the canonicalization is kinda counter-intuitive. I'll try to release a new version of refa by the end of the week.