Provide support for re.fullmatch in String trait type

A minor feature request: for `String` validation with a regex, `re.fullmatch` is more often going to be what we want to use than `re.match`. It would be nice to provide a way to do that full match.

Locate the String trait's regex validation entry point and the existing behavior based on re.match; no file or test path is identified in the issue. Read nearby validation tests, add coverage for full-match behavior, and consider the work done when String validation offers a way to use re.fullmatch without breaking existing matching.

Written by the indexing model from the issue text.