Review and update PlainDate and PlainDateTime API

[nekevss]

This PR reviews and makes updates to the API of PlainDate and PlainDateTime.

There are a variety of changes made:

• with_calendar methods made infallible
• PDT::to_plain_date and PDT::to_plain_time made infallible.
• Updates function signatures for TimeZone to not take a reference (in temporal_rs public API only, omitted from temporal_capi).
• Documentations updates.
• module renames from date.rs and datetime.rs to plain_date.rs and plain_date_time.rs, respectively.