This generalizes the work previously done for Promise job callbacks. There is now a microtask queue that correctly processes all queued microtasks after each turn of the event loop, as well as after a scripted callback finishes executing, and after a classic script executes.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#4283
- [X] There are tests for these changes
Source-Repo: https://github.com/servo/servo
Source-Revision: cbcafd18f4cb5973948b081b4c104d99735e2789
