Implement Regexp::RequiredPrefixUnanchored().

Change-Id: I1adcb0f77c28c5b54cf0cee2abe56f67a24c96bc
Reviewed-on: https://code-review.googlesource.com/c/re2/+/56212
Reviewed-by: Paul Wankadia <junyer@google.com>
3 files changed