regex/uregex API to set more state simultaneously

Proposal for uregex/regex API to set both the region and match start position independently, so the position at which a subsequent match operation begins can be a value other than start of region.
Useful when a long-running search has had to be interrupted (via the recent FindProgressCallback mechanism in uregex) and the search is later re-issued. No need to start the match from the beginning of the region again.

regex.h
void uregex_setRegionAndStart(URegularExpression *regexp, int64_t regionStart, int64_t regionLimit, int64_t startIndex, UErrorCode *status);
uregex.h
virtual RegexMatcher &region(int64_t regionStart, int64_t regionLimit, int64_t startIndex, UErrorCode &status);