[8.x] Add singular syntactic sugar to wormhole

[dwightwatson]

This adds syntactic sugar to the Wormhole for when using singular values. I've found that a number of my tests only go back or forward 1 unit of time and so this reads better.

// Before
$this->travel(1)->weeks();
$this->travel(-1)->weeks();

// After
$this->travel(1)->week();
$this->travel(-1)->week();

Each existing unit gets a singular alias that defers back to the original.

I've copied the docblocks as-is - if I updated them to say "Travel forward a single millisecond" it wouldn't be technically correct. But happy to amend as you see fit.