Auto merge of #15189 - jdm:microtasks, r=nox

Implement microtask checkpoints

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

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components/script/lib.rs

@@ -109,6 +109,7 @@ mod dom;
 pub mod fetch;
 pub mod layout_wrapper;
 mod mem;
+mod microtask;
 mod network_listener;
 pub mod origin;
 pub mod script_runtime;