Move custom elements stuff to their own dom::bindings submodule
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r? jdm
Moved the following functions from components/script/dom/bindings/interface.rs to a new file components/script/dom/bindings/htmlconstructor.rs:
- html_constructor()
- get_constructor_object_from_local_name()
- pop_current_element_queue()
- push_new_element_queue()
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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#19179.
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- [X] These changes do not require tests
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07fd6155aa
