Mark _serviceBrand properties using declare (#99312)

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

Also adds readonly

src/vs/workbench/api/common/extHostInitDataService.ts

@@ -9,6 +9,6 @@ import { createDecorator } from 'vs/platform/instantiation/common/instantiation'
 export const IExtHostInitDataService = createDecorator<IExtHostInitDataService>('IExtHostInitDataService');
 
 export interface IExtHostInitDataService extends Readonly<IInitData> {
-	_serviceBrand: undefined;
+	readonly _serviceBrand: undefined;
 }