fix: add check that $ref value is a string (asyncapi#92)

Co-authored-by: Maciej Urbańczyk <[email protected]>

tests/lib/index.spec.ts

@@ -1,4 +1,4 @@
-import { describe, expect, test, jest } from '@jest/globals';
+import { describe, expect, test } from '@jest/globals';
 import bundle from '../../src';
 import fs from 'fs';
 import path from 'path';