Improve error validation

Author: ehmicky

src/main.ts

@@ -9,6 +9,11 @@ export type { Options }
 const getOptions = (options: Options = {}) => {
   // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
   validateOptions(options)
+
+  if (options.classes !== undefined) {
+    throw new TypeError('"classes" must not be defined.')
+  }
+
   return options
 }