chore: Editor 'plugin' -> 'extension'

They've always been called extensions, not sure why the folder was plugins. Part of ongoing spring cleaning

shared/editor/extensions/Multiplayer.ts

@@ -0,0 +1,56 @@
+import {
+  ySyncPlugin,
+  yCursorPlugin,
+  yUndoPlugin,
+  undo,
+  redo,
+} from "@getoutline/y-prosemirror";
+import { keymap } from "prosemirror-keymap";
+import * as Y from "yjs";
+import Extension from "../lib/Extension";
+
+export default class Multiplayer extends Extension {
+  get name() {
+    return "multiplayer";
+  }
+
+  get plugins() {
+    const { user, provider, document: doc } = this.options;
+    const type = doc.get("default", Y.XmlFragment);
+
+    const assignUser = (tr: Y.Transaction) => {
+      const clientIds = Array.from(doc.store.clients.keys());
+
+      if (
+        tr.local &&
+        tr.changed.size > 0 &&
+        !clientIds.includes(doc.clientID)
+      ) {
+        const permanentUserData = new Y.PermanentUserData(doc);
+        permanentUserData.setUserMapping(doc, doc.clientID, user.id);
+        doc.off("afterTransaction", assignUser);
+      }
+    };
+
+    // only once we have authenticated successfully do we initalize awareness.
+    // we could send this earlier, but getting authenticated faster is more important
+    provider.on("authenticated", () => {
+      provider.setAwarenessField("user", user);
+    });
+
+    // only once an actual change has been made do we add the userId <> clientId
+    // mapping, this avoids stored mappings for clients that never made a change
+    doc.on("afterTransaction", assignUser);
+
+    return [
+      ySyncPlugin(type),
+      yCursorPlugin(provider.awareness),
+      yUndoPlugin(),
+      keymap({
+        "Mod-z": undo,
+        "Mod-y": redo,
+        "Mod-Shift-z": redo,
+      }),
+    ];
+  }
+}