Set `currentScript` to `null` for module scripts
I misunderstood the test case when I worked on #23545. That test case is
actually not related to dynamic import; instead, the reason why it
crashes is, `currentScript` should be updated to `null`.
In spec, the step 6 of [execute-the-script-block](https://html.spec.whatwg.org/multipage/scripting.html#execute-the-script-block) only says `Assert: document's currentScript attribute is null.` but doesn't says it should be set to null. Not sure if it can be improved.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)
- [x] There are tests for these changes
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