src/vs/workbench/contrib/webview/browser/webviewElement.ts/IFrameWebview/on
class IFrameWebview:	protected on<T = unknown>(channel: WebviewMessageChannels, handler: (data: T) => void): IDisposable {
		return addDisposableListener(window, 'message', e => {
			if (!e || !e.data || e.data.target !== this.id) {
				return;
			}
			if (e.data.channel === channel) {
				handler(e.data.data);
			}
		});
	}