Finalize the proposed webview serialization API: https://github.com/Microsoft/vscode/blob/245155d1614ce3cec37738c984369b8b03c4c32f/src/vs/vscode.proposed.d.ts#L291

Open questions:

• How does this interact with Capturing state of a web view when it becomes invisible #48542? Should the state from the webview be the state we persist?