Add platform-provided mixins explainer

[anaskim]

V1 includes:

• Rewording on interoperability.
• Adds examples on current workarounds used by frameworks.
• Adds goal to clarify the general and specific objectives of the proposal.
• "behavior" -> "mixin", to avoid confusion with "behaviour" spelling.
• Simplifies to add mixins to attachInternals() and avoid dynamic changes at runtime.
• Adds mixins property to ElementInternals to query behaviors.
• Removed "Open questions" section and expanded "Future Work" section with a "collision handling" note and 2 more proposed mixins (input and table behaviors).
• Updated "Accessibility, Security, and Privacy Considerations" section.
• Updated acknowledgements.