Upgrade eslint-plugin-matrix-org to 1.2.1

This upgrade came with a number of new lints that needed to be fixed across the code base. Primarily: explicit return types on functions, and explicit visibility modifiers on class members.

src/room/usePageUnload.ts

@@ -18,11 +18,11 @@ import { useEffect } from "react";
 
 import { platform } from "../Platform";
 
-export function usePageUnload(callback: () => void) {
+export function usePageUnload(callback: () => void): void {
   useEffect(() => {
     let pageVisibilityTimeout: ReturnType<typeof setTimeout>;
 
-    function onBeforeUnload(event: PageTransitionEvent) {
+    function onBeforeUnload(event: PageTransitionEvent): void {
       if (event.type === "visibilitychange") {
         if (document.visibilityState === "visible") {
           clearTimeout(pageVisibilityTimeout);