feat: add widgets schema validation (#3841)

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

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

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

@@ -0,0 +1,27 @@
+export default {
+  properties: {
+    minimal: { type: 'boolean' },
+    buttons: {
+      type: 'array',
+      items: {
+        type: 'string',
+        enum: [
+          'bold',
+          'italic',
+          'code',
+          'link',
+          'heading-one',
+          'heading-two',
+          'heading-three',
+          'heading-four',
+          'heading-five',
+          'heading-six',
+          'quote',
+          'bulleted-list',
+          'numbered-list',
+        ],
+      },
+    },
+    editorComponents: { type: 'array', items: { type: 'string' } },
+  },
+};