chore: Move to Typescript (#2783)

This PR moves the entire project to Typescript. Due to the ~1000 ignores this will lead to a messy codebase for a while, but the churn is worth it – all of those ignore comments are places that were never type-safe previously.

closes #1282

app/multiplayer/MultiplayerExtension.ts

@@ -1,4 +1,3 @@
-// @flow
 import { keymap } from "prosemirror-keymap";
 import { Extension } from "rich-markdown-editor";
 import {
@@ -19,7 +18,7 @@ export default class MultiplayerExtension extends Extension {
     const { user, provider, document: doc } = this.options;
     const type = doc.get("default", Y.XmlFragment);
 
-    const assignUser = (tr) => {
+    const assignUser = (tr: Y.Transaction) => {
       const clientIds = Array.from(doc.store.clients.keys());
 
       if (