Support native forms

[maxatdetroit]

The Problem

ShadowDOM encapsulation means 3rd party javascript and (maybe?) standard use-agent form submission logic are unable to fetch the relevant form data needed during submission events. This is likely the issue in #1460 and seems to be corroborated by other design systems but still needs to be confirmed.

The Solution

Each form control component in the design system needs to implement the formdata event to append their data to the form just prior to submission.