feat: multiple units support in toRelative

[andreialecu]

This has been requested before in #579, see discussion there for more details.

My use case is that during the first minute, .toRelative() will print something like 1 second ago. I'd like to avoid updating it every second (for performance purposes), and instead I'd like it to print out 0 minutes ago. This way I can schedule updates once every minute instead of every second.

This PR adds a way to override which units are considered when formatting a relative date.

See the tests I added as well.

Fixes #579.

[andreialecu]

One thing to note is that this also fixes an issue with rounding:

I added a test for it:

  expect(base.minus({ seconds: 1 }).toRelative({ base, unit: "minutes" })).toBe("0 minutes ago");

On master, that printed in 0 minutes which was wrong.