YAML.visit() should support async visitors

[eemeli]

Is your feature request related to a problem? Please describe.
Custom tags need to be sync, so that e.g. YAML.parse() may stay synchronous. Asynchronous tag resolution therefore needs a different solution.

Describe the solution you'd like
It should be possible to return a promise from a visitor, and have that "just work".

Additional context
Original issue raised by @suerta-git in discussion #359.