Add gap_info_from_local_datetime to get information about a gap

[acrrd]

The function returns information such as the start, offset before the beginning, and the end of the gap.
This will help handle cases where TimeZone::from_local_datetime returns LocalResult::None as discussed in #183 and other use cases mentioned in chronotope/chrono#1448.

This PR doesn't want to implement a variant of from_locale_datetime that returns more information when there is a gap because this interface will be defined in the discussion of #1448.

Close #183

[acrrd]

Rebased on main to make CI pass

[djc]

Thanks!