Implement missing "insert a foreign element" parsing algorithm

[jdm]

whatwg/html#4290

[AnshulMalik]

@jdm Can you provide some more context here please?

[jdm]

It took me a while to read and understand what it was about, but the fundamental change is this commit. From reading Servo's html5 parser implementation, it looks like it's modifying an algorithm that has changed since we originally implemented it. I think I see a few important steps here - currently, the html5ever parser implements the "create an element for the token" algorithm here (spec), but we're missing the "insert a foreign element" algorithm that the HTML specification PR changed (https://html.spec.whatwg.org/multipage/parsing.html#insert-a-foreign-element). We should:

• modify html5ever's insert_element function to implement the "insert a foreign element" algorithm (which invokes the "create an element for the token", which we can mostly reuse)
• add push_new_element_queue and pop_current_element_queue methods to the TreeSink trait (here)
• implement these new methods in Servo's parser implementation, checking self.parsing_algorithm and calling the appropriate ScriptThread methods (push_new_element_queue and pop_current_element_queue)

I believe those changes should allow us to pass some tests that are currently failing.

[jdm]

The test to run would be ./mach test-wpt tests/wpt/web-platform-tests/custom-elements/connected-callbacks-html-fragment-parsing.html.

[jdm]

And Servo's parser implementation lives in components/script/dom/servoparser/mod.rs.

[AnshulMalik]

Thanks @jdm so much. I'll take a stab at it.