☂️ Add a new Dart platform interoperability overview

[parlough]

We do not have a singular page that documents and outlines Dart support for platform-specific interoperability. Now that interop support has had massive strides in the past few releases and HTML/JS support is being repositioned as a form of interop, it is time to have a consistent destination.

• [ ] Add a new page at dart.dev/interop
  • What is interoperability in terms of Dart
  • Why it exists and when it might be needed
  • Outline and index what forms of interoperability Dart supports
  • 🌟 Direct users to the appropriate solution for their platform and problem.
  • Perhaps what is still in progress?
  • Consider a cross reference to Flutter's pigeon docs
• [ ] Add text and cross-link from https://dart.dev/overview
  • Should clearly and consisely call out Dart's great interop support (slightly marketing style)
• [ ] Consider a specific call out or cross-link from homepage

[parlough]

I'm going to assign @MaryaBelanger as she'll likely be writing this since she is currently working with engineers on the platform-specific interop docs.

I'm also going to assign @mit-mit for how we want to position Dart's overall interop story and what's important to make sure is covered, or shouldn't be covered yet.

Please feel free to tag others or reassign as needed. Thank you!