The DOM spec defines what it means for an element to be an "editing
host", and the Editing spec does the same for the "editable" concept.
Replace our `Node::is_editable()` implementation with these
spec-compliant algorithms.
An editing host is an element that has the properties to make its
contents effectively editable. Editable elements are descendants of an
editing host. Concepts like the inheritable contenteditable attribute
are propagated through the editable algorithm.
I believe this is an error in the UI Events spec, and it should be
updated to match the HTML spec (which uses WindowProxy everywhere).
This fixes a bunch of issues already covered by existing WPT tests.
Spec bug: https://github.com/w3c/uievents/issues/388
Note that WebKit has been using WindowProxy instead of Window in
UI Events IDL since 2018:
816158b4aa
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
