Fix compiler error related to DOMPurify

DOM on the server and on the client are different, so a wrapper
around the DOMPurify library call is needed. This commit installs
the isomorphic dompurify package, which solves the compiler error.

Author: WZWren

peerprep/app/questions/[question]/[roomID]/question.tsx

@@ -7,7 +7,7 @@ import styles from "@/style/question.module.css";
 import { useRouter } from "next/navigation";
 import { deleteQuestion } from "@/app/api/internal/questions/helper";
 import CollabEditor from "@/components/questionpage/CollabEditor";
-import DOMPurify from "dompurify";
+import DOMPurify from "isomorphic-dompurify";
 
 interface Props {
   question: Question;

peerprep/app/questions/[question]/question.tsx

@@ -7,7 +7,7 @@ import styles from "@/style/question.module.css";
 import { useRouter } from "next/navigation";
 import { deleteQuestion } from "@/app/api/internal/questions/helper";
 import CollabEditor from "@/components/questionpage/CollabEditor";
-import DOMPurify from "dompurify";
+import DOMPurify from "isomorphic-dompurify";
 
 interface Props {
   question: Question;