Support the RegExp `v` flag

[BenjaminAster]

The RegExp v flag (a.k.a. RegExp.prototype.unicodeSets) is now in the main ECMAScript standard and supported across all modern browsers, but TypeScript/JavaScript syntax highlighting breaks when using it. Supporting it should work similarly to #862.

(see also https://tc39.es/ecma262/multipage/text-processing.html#sec-regexpinitialize)