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/MaxLength.ts

@@ -0,0 +1,23 @@
+import { Plugin, Transaction } from "prosemirror-state";
+import Extension from "../lib/Extension";
+
+export default class MaxLength extends Extension {
+  get name() {
+    return "maxlength";
+  }
+
+  get plugins() {
+    return [
+      new Plugin({
+        filterTransaction: (tr: Transaction) => {
+          if (this.options.maxLength) {
+            const result = tr.doc && tr.doc.nodeSize > this.options.maxLength;
+            return !result;
+          }
+
+          return true;
+        },
+      }),
+    ];
+  }
+}