RegExp v flag tracking issue

[bnoordhuis]

https://github.com/tc39/proposal-regexp-v-flag is a number of things that I'll break out into separate pull requests:

1. the flag and patterns that turn into early SyntaxErrors, ex. /[a-]/v (accepted by u but not v)

2. properties of strings, that is, code point sequences; ex. /\p{RGI_Emoji}/v - what https://github.com/tc39/proposal-regexp-unicode-sequence-properties was advocating before it got merged into the regexp-v-flag proposal

3. set union and difference, ex. /\p{Format}&&\p{Control}/v and /[\p{Decimal_Number}--[0-9]]/v (also nested character classes)