Convert many simple getters for events into readonly properties

We seem to use the readonly property way of exposing these values more widely. Standardizing this a bit across the codebase

src/vs/workbench/api/browser/mainThreadComments.ts

@@ -57,7 +57,7 @@ export class MainThreadCommentThread implements modes.CommentThread {
 	}
 
 	private _onDidChangeLabel = new Emitter<string>();
-	get onDidChangeLabel(): Event<string> { return this._onDidChangeLabel.event; }
+	readonly onDidChangeLabel: Event<string> = this._onDidChangeLabel.event;
 
 	private _comments: modes.Comment[] | undefined;