Refactor micro time to use a rounded duration

[keithamus]

This is a big refactor of the micro format, to remove our rather strange rounding logic and instead use Duration with some logic which is a bit easier to reason about (and potentially easier to delete).

The main benefit here is that:

1. micro format is now like elapsed but with some extra logic. This pairs down a lot of the complexity we had with the format.
2. The micro format is now somewhat easier to deprecate, which is our intent with v5.
3. The format relies more on our Duration object which is a ponyfill for Temporal.Duration, and so one day can be deleted (perhaps a v6 version 😉)
4. This lays ground work for us to better refactor the timeAgo and timeUntil functions which just need to be wrangled to use the same rounding logic.

It should be noted that this does change some tests! Mostly around the ways with which the micro format gets displayed. Units are rounded up in what might be considered more expected ways. Week units are introduced (so 4d, 5d, 6d, 7d will all now become 1w). Days are rounded to months, so 31d is 1m, and days are now rounded to years (so 364d becomes 1y).

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