Add index.ts for matrixrtc module

[toger5]

This is required so we can (finally) remove the react-sdk import eslint exception comments.

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

[fkwp]

lgtm