feat: add widgets schema validation (#3841)

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

@@ -1,10 +1,12 @@
 import controlComponent from './NumberControl';
 import previewComponent from './NumberPreview';
+import schema from './schema';
 
 const Widget = (opts = {}) => ({
   name: 'number',
   controlComponent,
   previewComponent,
+  schema,
   ...opts,
 });