Add platform-provided mixins explainer (#1208)

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.

Author: anaskim

.github/ISSUE_TEMPLATE/platform-provided-mixins.md

@@ -0,0 +1,7 @@
+---
+name: Platform-Provided Mixins
+about: new issue
+title: "[Platform-provided mixins] <TITLE HERE>"
+labels: PlatformProvidedMixins
+assignees: anaskim
+---