Switch to standard Promises

[jason0x43]

Intern 4.x makes extensive use of a Task class that provides cancellation and finally. Now that finally is part of standard promises, and given that cancellation isn't all that useful, the Task class should be replaced with Promise, and all APIs returning Tasks should now return PromiseLike.

The webdriver Command class should remain a custom Promise, but based on Promise rather than Task / CancellablePromise.

[jason0x43]

A somewhat simpler way to implement cancellation that Intern currently does: https://gist.github.com/devpaul/140f58f8787306798a058c515db260d2

This could be useful in places where cancellation is useful.

[jason0x43]

This is also necessary if we want to switch to async/await (which would make the code much easier to follow in many places) since async functions have to return native Promises.