v3 rewrite

[isaacs]

• Create two classes, Touch and TouchSync. Override just the fs-calling bits to swap out the sync-ness. (Pattern established in minipass, minizlib, and tar.)
• Async method returns a promise, attaches the cb to it if provided.
• handle directories (or at least, error properly)
• Don't use nopt in the bin
• bin takes a timestamp like touch(1)
• tap --100 test coverage