Format constant/enum

It seems that there are no `enum` to apply formats.
We must use strings everytime (e.g. `.format("L")`.
IIt would be nice to have already made values, for instance, (like Luxon allows) `.format(DateTime.DATE_SHORT)` -> "01/22/22".

Start by reviewing Day.js's existing format API and the Luxon example in the issue to determine which named constants are in scope. Define the intended constant names and output values, then verify that formatting with each constant produces the documented result.

Written by the indexing model from the issue text.