test: add test for isExternal

Author: sy-records

Diff for: test/unit/core-util.test.js

@@ -59,5 +59,13 @@ describe('core/util', () => {
 
       expect(result).toBeTruthy();
     });
+
+    test('external url with two \\ ', () => {
+      const result = isExternal(
+        '/\\\\example.github.io/docsify/demo.md'
+      );
+
+      expect(result).toBeTruthy();
+    });
   });
 });