Auto merge of #25515 - pshaughn:checkpoints, r=jdm

Add microtask checkpoints to script elements and custom elements

Servo had a microtask checkpoint at the end of running a script, but there was also supposed to be one at the end of HTML-parsing a script element before Javascript-parsing the script itself, and there were supposed to be checkpoints immediately after the call to a custom element constructor. This adds those, passing all cases of one WPT test file.

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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 #25016 except for the remaining not-really-about-microtasks case #25514

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- [X] There are tests for these changes

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