feat: add widgets schema validation (#3841)

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' } },
+    },
+  },
+};