Broaden use of SearchValues in TryFindNextPossibleStartingPosition in Regex (#89205)

Broaden use of SearchValues in TryFindNextPossibleStartingPosition in Regex (#89205)

SearchValues has been updated to have an ASCII fast-path for inputs that are not only ASCII.  This means we can simplify TryFindNextPossibleStartingPosition in Regex to not track AsciiSet specially and instead just increase the number of characters we query the set for (from 5 to 128).  That way, we'll use SearchValues rather than emitting our own helper up until a (semi-arbitrary) point where we deem it impossible or infeasible to enumerate all the chars that make up the set.