Mark _serviceBrand properties using declare (#99312)

_serviceBrand is only used for typing and should not result in emit

Also adds readonly

src/vs/code/electron-browser/sharedProcess/sharedProcessMain.ts

@@ -95,7 +95,7 @@ class MainProcessService implements IMainProcessService {
 		private mainRouter: StaticRouter
 	) { }
 
-	_serviceBrand: undefined;
+	declare readonly _serviceBrand: undefined;
 
 	getChannel(channelName: string): IChannel {
 		return this.server.getChannel(channelName, this.mainRouter);