[FR] Composition cancellation

[StrahilKazlachev]

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 <compose> inserts contents twice templating-resources#335 could be solved way cleaner.
  We should also check other parts of the framework where cancellation will be appropriate.