LibWeb: Split the qualified name if there is a prefix

[ananas-dev]

This fixes (at least) one WPT test and I imported it :)

The expected behavior is described here in the spec:
https://dom.spec.whatwg.org/#concept-element-local-name

It was already implemented correctly when constructing a QualifiedName in Element::validate_and_extract.

I don't know if my sequence of string conversions is the most efficient though, but don't hesitate to comment if there is a better one so that I can learn my way around AK.

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[tcl3]

Thanks for working on this! Sorry it's taken a while to get to. There is currently a conflict that needs to be resolved.

[ananas-dev]

Thanks for working on this! Sorry it's taken a while to get to. There is currently a conflict that needs to be resolved.

Thanks for the review :)
I changed the code to use DOM::validate_and_extract and added the error handling you suggested. I also added the same error handling to the DOM::create_element call right below since I figured I would make sense.

[tcl3]

Looks good! Please use the "Resolve conversation" button when you've dealt with review feedback.