Proposal: Common public interface for ElementInternals

There's no standard API to expose `ElementInternals` to `@public`. Third-party tooling like Automated Accessibility Testing tools are unable to reliably "guess" and find those exposed internals (e.g. cannot find `aria-role`). It would be good for the Web Components Community Group to have an agreed upon standard for exposing `ElementInternals` so other tooling can build from our convention.