refactor: convert function expressions to declarations (#4926)

packages/netlify-cms-widget-string/src/index.js

@@ -1,12 +1,14 @@
 import controlComponent from './StringControl';
 import previewComponent from './StringPreview';
 
-const Widget = (opts = {}) => ({
-  name: 'string',
-  controlComponent,
-  previewComponent,
-  ...opts,
-});
+function Widget(opts = {}) {
+  return {
+    name: 'string',
+    controlComponent,
+    previewComponent,
+    ...opts,
+  };
+}
 
 export const NetlifyCmsWidgetString = { Widget, controlComponent, previewComponent };
 export default NetlifyCmsWidgetString;