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/SmartText.ts

@@ -0,0 +1,28 @@
+import { ellipsis, smartQuotes, InputRule } from "prosemirror-inputrules";
+import Extension from "../lib/Extension";
+
+const rightArrow = new InputRule(/->$/, "→");
+const oneHalf = new InputRule(/1\/2$/, "½");
+const threeQuarters = new InputRule(/3\/4$/, "¾");
+const copyright = new InputRule(/\(c\)$/, "©️");
+const registered = new InputRule(/\(r\)$/, "®️");
+const trademarked = new InputRule(/\(tm\)$/, "™️");
+
+export default class SmartText extends Extension {
+  get name() {
+    return "smart_text";
+  }
+
+  inputRules() {
+    return [
+      rightArrow,
+      oneHalf,
+      threeQuarters,
+      copyright,
+      registered,
+      trademarked,
+      ellipsis,
+      ...smartQuotes,
+    ];
+  }
+}