feat: add widgets schema validation (#3841)

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

@@ -1,11 +1,13 @@
 import controlComponent from './ListControl';
 import NetlifyCmsWidgetObject from 'netlify-cms-widget-object';
+import schema from './schema';
 
 const previewComponent = NetlifyCmsWidgetObject.previewComponent;
 const Widget = (opts = {}) => ({
   name: 'list',
   controlComponent,
   previewComponent,
+  schema,
   ...opts,
 });
 

packages/netlify-cms-widget-list/src/schema.js

@@ -0,0 +1,9 @@
+export default {
+  properties: {
+    allow_add: { type: 'boolean' },
+    collapsed: { type: 'boolean' },
+    summary: { type: 'string' },
+    minimize_collapsed: { type: 'boolean' },
+    label_singular: { type: 'string' },
+  },
+};