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.

---
- [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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tests/wpt/metadata/html/webappapis/scripting/event-loops/microtask_after_raf.html.ini

@@ -1,6 +0,0 @@
-[microtask_after_raf.html]
-  type: testharness
-  disabled: https://github.com/servo/servo/issues/13501
-  [Microtask execute immediately after script]
-    expected: FAIL
-

tests/wpt/metadata/html/webappapis/scripting/event-loops/microtask_after_script.html.ini

@@ -1,5 +0,0 @@
-[microtask_after_script.html]
-  type: testharness
-  [Microtask immediately after script]
-    expected: FAIL
-

tests/wpt/metadata/html/webappapis/scripting/event-loops/task_microtask_ordering.html.ini

@@ -1,8 +1,5 @@
 [task_microtask_ordering.html]
   type: testharness
-  [Basic task and microtask ordering]
-    expected: FAIL
-
   [Level 1 bossfight (synthetic click)]
     expected: FAIL
-
+    bug: https://github.com/servo/servo/issues/1980