feat: add widgets schema validation (#3841)

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

@@ -1,10 +1,12 @@
 import controlComponent from './CodeControl';
 import previewComponent from './CodePreview';
+import schema from './schema';
 
 const Widget = (opts = {}) => ({
   name: 'code',
   controlComponent,
   previewComponent,
+  schema,
   allowMapValue: true,
   codeMirrorConfig: {},
   ...opts,

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

@@ -0,0 +1,11 @@
+export default {
+  properties: {
+    default_language: { type: 'string' },
+    allow_language_selection: { type: 'boolean' },
+    output_code_only: { type: 'boolean' },
+    keys: {
+      type: 'object',
+      properties: { code: { type: 'string' }, lang: { type: 'string' } },
+    },
+  },
+};