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/PreventTab.ts

@@ -0,0 +1,15 @@
+import Extension, { Command } from "../lib/Extension";
+
+export default class PreventTab extends Extension {
+  get name() {
+    return "preventTab";
+  }
+
+  keys(): Record<string, Command> {
+    return {
+      // No-ops prevent Tab escaping the editor bounds
+      Tab: () => true,
+      "Shift-Tab": () => true,
+    };
+  }
+}