Introducing @epic-web/invariant

[kentcdodds]

The two invariant utilities have been extracted into a new package called @epic-web/invariant! Also with the improvement that it throws a custom InvariantError now so you can distinguish them from other types of errors (could be useful in some cases I suppose).

Find the commit here: 5164851

Star the package here: https://github.com/epicweb-dev/invariant

Cheers!