clarify variable name, add docstring for isBuiltin

Author: lobsterkatie

packages/utils/src/is.ts

@@ -23,9 +23,15 @@ export function isError(wat: unknown): wat is Error {
       return isInstanceOf(wat, Error);
   }
 }
-
-function isBuiltin(wat: unknown, ty: string): boolean {
-  return objectToString.call(wat) === `[object ${ty}]`;
+/**
+ * Checks whether given value is an instance of the given built-in class.
+ *
+ * @param wat The value to be checked
+ * @param className
+ * @returns A boolean representing the result.
+ */
+function isBuiltin(wat: unknown, className: string): boolean {
+  return objectToString.call(wat) === `[object ${className}]`;
 }
 
 /**