Have formatter support timestamp fields

Problem/Motivation

Since Smart Date already works with timestamps, it should have no problem supporting timestamp fields. It wouldn't necessarily provide a ton of extra value (range formatting isn't useful here) but it could be helpful to easily have the formatting consistent across different kinds of fields.

While we're at it, we should also support the Publication Date module, which defines a new field type of published_at and is also compatible with timestamps.

Proposed resolution

Update Smart Date's default formatter to support timestamp and published_at fields, and adjust the conditional logic within the viewElements method of SmartDateTrait to properly set the $start_ts and $end_ts values to the field's timestamp value.