Marking most private emitters as readonly

Emitters should generally not be reassigned as their corresponding event would also be lost. Marking these as readonly just to enforce this

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

@@ -239,7 +239,7 @@ export class ExtHostCommentThread implements vscode.CommentThread {
 		return this._uri;
 	}
 
-	private _onDidUpdateCommentThread = new Emitter<void>();
+	private readonly _onDidUpdateCommentThread = new Emitter<void>();
 	readonly onDidUpdateCommentThread = this._onDidUpdateCommentThread.event;
 
 	set range(range: vscode.Range) {