Implement document response correctly for XHR

[KiChjang]

Code: components/script/dom/xmlhttprequest.rs
Spec: https://xhr.spec.whatwg.org/#document-response
Test: tests/wpt/web-platform-tests/XMLHttpRequest/responsexml-*.htm

cc @liviuba

[liviuba]

I'd really like to take this one

[Ms2ger]

It's not clear to me there's anything to be done here that we can do right now.

[KiChjang]

@Ms2ger, @liviuba in their pull request for extracting document_response into its own function said that they would like to work on making step 6 match the spec.

[Ms2ger]

Do we have infra for prescanning?

[KiChjang]

Can we use parsell or tendrill for this?

[jdm]

@liviuba Have you made any progress here?

[liviuba]

@jdm Got stuck with Parsell because of the lack of documentation / usage in general / experience with parsers on my side. Right now I'm trying to at least partially implement prescanning with Nom and once I get that going moving to Parsell should be doable

[jdm]

It's not clear to me that this is a good E-easy issue due to the open questions around how to implement prescanning, so I'm removing the label.