src/vs/workbench/contrib/webview/browser/webviewElement.ts/WebviewElement/on
class WebviewElement:	private on<K extends keyof FromWebviewMessage>(channel: K, handler: (data: FromWebviewMessage[K], e: MessageEvent) => void): IDisposable {
		let handlers = this._messageHandlers.get(channel);
		if (!handlers) {
			handlers = new Set();
			this._messageHandlers.set(channel, handlers);
		}

		handlers.add(handler);
		return toDisposable(() => {
			this._messageHandlers.get(channel)?.delete(handler);
		});
	}