Support code datetime & timestamp field types

Problem/Motivation

Now that we can turn things like "7:00pm" into "7pm" (see #2959067: Option to conditionally drop minutes (when :00) and am/pm from time), there is a use case for this formatter to work with individual datetime/timestamp fields as well as ranges.

Proposed resolution

Add datetime and timestamp to the list of field types supported by the daterange_compact field formatter.

Remaining tasks

Code + test coverage.

User interface changes

None.

API changes

None. The formatter service already copes with identical start and end values. It considers these as one date or timestamp and doesn't try to render it as a range.

Data model changes

None.