If a node with `contenteditable=true/plaintextonly` is the child of an
editable node or an editing host, we should make it editable instead of
an editing host. This effectively merges nested editing hosts together,
which is how other browsers deal with this as well.
Gains us 5 WPT subtest passes in `editing`.
We currently delete a single code unit. If the user presses backspace on
a multi code point emoji, they are going to expect the entire emoji to
be removed. This now matches the behavior of Chrome and Firefox.
Anchor the minimum functionality for this. WPT has an extensive suite
to test editing functionalities, but they all take a long time to
execute - so let's have a simple regression test in-tree for now.
