LibWeb: Change IDL attribute type to USVString where applicable

[tcl3]

This PR changes some IDL attribute return types to USVString, where the specification says that's what should be used and updates the bindings generator to ensure that reflected USVString attributes behave correctly.

USVString attributes that represent a URL are now explicitly marked with the URL extended attribute, as the specification says that they are supposed to behave differently to attributes that don't represent a URL.

Fixes 5 subtests on: http://wpt.live/html/dom/usvstring-reflection.https.html

[awesomekling]

Reflected USVString attributes now have any surrogates with U+FFFD and any USVString attributes that are URLs are now percent encoded.

I'm struggling to parse this. :thonk:

[tcl3]

I'm struggling to parse this. :thonk:

I've got no idea how I mangled that so badly! Hopefully, it should now make sense.

I also found a few more places where USVString should be used, so I've updated those places too. I had to update a few tests due to these changes. The test expectations now match what is generated by other browsers in these cases.

[jamierocks]

Fixes 5 subtests on: http://wpt.live/html/dom/usvstring-reflection.https.html

This probably fixes more tests - I've noticed quite a few issues related to attributes that are URLs :)

[tcl3]

I've done some more testing on this and the set of USVString attributes that are marked as containing a URL seems to vary across browsers. I've updated the PR so that we only mark an attribute as containing a URL if that's what's done by all the major engines.