HTMLInputElement now inherits from FormAssociatedElement, which will
be a common base for the handful of elements that need to track their
owner form (and register with it for the form.elements collection.)
At the moment, the owner form is assigned during DOM insertion/removal
of an HTMLInputElement. I didn't implement any of the legacy behaviors
defined by the HTML parsing spec yet.
Text <input> fields will now generate a basic shadow DOM and attach it
to the input element.
The shadow DOM contains a <div> with some inline style, and an always-
editable text node inside it. Accessing the "value" attribute on such
an input element will get/set the value from that text node.
This is really cool, although not super stable since HTML editing is
not super stable. But it's a start! :^)
