Re-implement async config option

[eddyerburgh]

What problem does this feature solve?

Vue Test Utils has a synchronous API by walking all watchers and setting them to sync, but this implementation is buggy.

Reimplementing an async private config option would solve this problem for Vue Test Utils.

What does the proposed API look like?

Add an async config option that would be used when watchers are created.

The same option that was removed in this commit.

I'm happy to make a PR to add this.