Implement `regexec2` using `stri_locate_first_regex`

`stri_locate_first_regex` does not distinguish between a non-existing capture group and a no-match to a capture
group in the case of many patterns. Also, it does not return capture group names.

Until https://github.com/gagolews/stringi/issues/424 and/or https://github.com/gagolews/stringi/issues/127 is resolved,
the current version of `regexec2` must be implemented based on `stri_locate_all_regex`, and this includes some performance penalty.

Start with the existing regexec2 entry point and compare the results from stringi's stri_locate_all_regex and stri_locate_first_regex. Review stringi issues #424 and #127 for the upstream limitations. Done means regexec2 handles nonexistent versus unmatched capture groups and returns capture-group names, with the documented performance tradeoff.

Written by the indexing model from the issue text.