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.
Instead of recursively iterating all descendants of the common ancestor
of the new line range that are not contained by that range, skip the
entire node tree as soon as we determine they're not.
In all these cases there should be an ancestor available, but it
definitely cannot hurt to be a bit more defensive about this and prevent
nullptr dereferences.
To facilitate the implementation of "delete" and all associated
algorithms, split off this piece of `Document` into a separate
directory.
This sets up the infrastructure for arbitrary commands to be supported.
