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/History.ts

@@ -0,0 +1,22 @@
+import { history, undo, redo } from "prosemirror-history";
+import { undoInputRule } from "prosemirror-inputrules";
+import Extension from "../lib/Extension";
+
+export default class History extends Extension {
+  get name() {
+    return "history";
+  }
+
+  keys() {
+    return {
+      "Mod-z": undo,
+      "Mod-y": redo,
+      "Shift-Mod-z": redo,
+      Backspace: undoInputRule,
+    };
+  }
+
+  get plugins() {
+    return [history()];
+  }
+}