Implement ES2018 Promise.prototype.finally in dojo/promise/Promise

[RoyTinker]

Supersedes #352

Implementing ES2018's Promise.prototype.finally in dojo/promise/Promise instead of dojo/Deferred because:

• Implementing in dojo/Deferred leaves an API hole -- we have an abstract "finally" method in dojo/promise/Promise that is left without an implementation in Promise instances built in dojo/_base/Deferred. So new baseDeferred().promise.finally(function(){}) would throw the "abstract" TypeError. Implementing in dojo/promise/Promise allows all Deferreds that use this Promise class to inherit finally in their promise objects -- dojo/Deferred, dojo/_base/Deferred, and even any (hypothetical?) custom Deferred classes built by users.
• finally can be implemented directly in terms of then with reduced complexity over the previous implementation - at runtime, computationally, and in the source.

Includes comprehensive, passing unit tests in tests/unit/promise/Promise.js.

[dylans]

Landing in master... cannot backport as it's a change in functionality.

[RoyTinker]

Thanks @dylans