Auto merge of #27100 - muodov:master, r=gterzian

Implement HTMLFormElement.requestSubmit()

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This PR contains an implementation of [HTMLFormElement.requestSubmit()](https://html.spec.whatwg.org/multipage/forms.html#dom-form-requestsubmit)

This is literally my first hundred lines of Rust code, so if I crossed a few sacred lines here and there, please go easy on me :)

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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 #23417

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- [x] [WPT tests](https://github.com/servo/servo/blob/master/tests/wpt/web-platform-tests/html/semantics/forms/the-form-element/form-requestsubmit.html) for these changes
There are two tests that still fail because we don't support `:invalid` CSS selector (see #10781). I verified that they pass if you change them to not use `:invalid`. Should be unlocked by #26729.

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