[FR] Composition cancellation

**I'm submitting a feature request**

* **Library Version:**
vNext

**Current behavior:**
No way to cancel an ongoing composition - `CompostionEngine.prototype.compose`

**Expected/desired behavior:**

* **What is the expected behavior?**
There should be a way to *request* a composition cancellation.
It is a pity that it is still unclear whether the `AbortController` is the way to go or there will be an *ES* defined mechanism.

* **What is the motivation / use case for changing the behavior?**
Issues like the following aurelia/templating-resources#335 could be solved way cleaner.
We should also check other parts of the framework where *cancellation* will be appropriate.

Start at CompostionEngine.prototype.compose, the entry point named in the issue, and review how ongoing compositions are tracked. Compare the requested cancellation behavior with AbortController and the possible ES mechanism, then identify other framework areas where cancellation is appropriate; done requires an agreed cancellation design and its implementation.

Written by the indexing model from the issue text.