feat: add widgets schema validation (#3841)

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

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

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

@@ -0,0 +1,8 @@
+export default {
+  properties: {
+    format: { type: 'string' },
+    dateFormat: { oneOf: [{ type: 'string' }, { type: 'boolean' }] },
+    timeFormat: { oneOf: [{ type: 'string' }, { type: 'boolean' }] },
+    pickerUtc: { type: 'boolean' },
+  },
+};