Add `MappedLocalTime::try_map`

[pitdicker]

Passing on one error in unix::Cache::offset requires a match that is very similar to the one in TimeZone::from_local_datetime. I added MappedLocalTime::try_map that returns MappedLocalTime::None that can be used in both places.

For the 0.5.x branch it will be easy to convert this method to return a Result instead.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 91.82%. Comparing base (3adfd88) to head (e8db5cd).

Files	Patch %	Lines
src/offset/mod.rs	73.33%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1533 +/- ## ======================================= Coverage 91.82% 91.82% ======================================= Files 40 40 Lines 18345 18345 ======================================= Hits 16846 16846 Misses 1499 1499 ```

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[pitdicker]

This PR on its own doesn't look well-motivated...

Not sure what you mean. Maybe as another motivation: to implement addition and substraction for a CalendarDuration (#1282) months and days should be added in localtime with a MappedLocalTime as result. Then seconds should be added using a method such as this so the type remains a MappedLocalTime.

But I have to admit I'm not sure if the current two uses in this PR for the 0.4 versions will remain useful for 0.5 with one of the plans in 1448#issuecomment-1987220921. So the method is pub(crate) for now.

[djc]

Ah, sorry, I missed that there are sort of two users for the new method. Looks a little surprising because one of the callers did not previously have the extensive logic implemented in the method.

[pitdicker]

Yes, that was the issue I set out to fix :smile:. Sorry for not saying so better.