Add toAlways, the inverse of toNever.

[younata]

toNever was added somewhat recently, where we verify that a matcher continues to fail until the timeout happens. It felt odd to me that there wasn't a way to verify that a watcher continues to succeed until the timeout happens. So I added toAlways to capture that.

This adjusts how the private async function works, to also take in the behavior that we want the predicate to always match. I briefly looked into consolidating AsyncMatchStyle.never and AsyncMatchStyle.always because they are very similar, but decided against DRY'ing that up because it would unnecessarily add complexity.

This also adds documentation in the README around how to use toNever and toAlways. It's not great documentation, but I hope this gets the idea across.

• Does this have tests?
• Does this have documentation?
• Does not break the public API.
• Is this a new feature (Requires minor version bump)?

[jessesquires]

Rebasing this to re-run CI