feat(core): Add new matchers and improve negation messaging

ts/namespaces/string.ts

@@ -47,4 +47,13 @@ export class StringMatchers {
       `Expected string to have length ${length}`
     );
   }
+  /**
+   * Alias for empty string check
+   */
+  toBeEmpty() {
+    return this.assertion.customAssertion(
+      (value) => (value as string).length === 0,
+      `Expected string to be empty`
+    );
+  }
 }