Use `v` regexp flag instead of `u`

[nicolo-ribaudo]

The v flags add more support to regular expression features:

• "set operation", such as [[a-z]--[dhk]] to express "the set a-z, excluding the set dhk (https://github.com/tc39/proposal-regexp-v-flag)
• multi-codepoint properties in \p, such as \p{RGI_Emoji_ZWJ_Sequence} (https://github.com/tc39/proposal-regexp-unicode-sequence-properties)

The HTML <input>'s pattern attribute has also been recently updated to use v instead of u (https://html.spec.whatwg.org/#compiled-pattern-regular-expression).

[bathos]

The “set operations” would be useful. I‘m pretty sure the second never will be, though, because ...

— the illusion that non-ASCII is matchable is limited to literal input: it converts non-ASCII input to percent encoded UTF-8, but regexp pattern components aren’t likewise “translated”, so expressing things like \p{RGI_Emoji_Sequence} seems to always require input that’s similar to what transpilers might produce today for engines that don’t support multi- or single- codepoint properties at all.