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/extHostTimeline.ts

@@ -22,7 +22,7 @@ export interface IExtHostTimeline extends ExtHostTimelineShape {
 export const IExtHostTimeline = createDecorator<IExtHostTimeline>('IExtHostTimeline');
 
 export class ExtHostTimeline implements IExtHostTimeline {
-	_serviceBrand: undefined;
+	declare readonly _serviceBrand: undefined;
 
 	private _proxy: MainThreadTimelineShape;