LibJS: Implement Set.prototype.isSubsetOf

Author: https://github.com/IdanHo
Commit: e29be4eaa8
Pull-request: https://github.com/SerenityOS/serenity/pull/16279
Reviewed-by: https://github.com/TobyAsE
Reviewed-by: https://github.com/davidot ✅
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Tests/builtins/Set/Set.prototype.isSubsetOf.js

@@ -0,0 +1,8 @@
+test("basic functionality", () => {
+    expect(Set.prototype.isSubsetOf).toHaveLength(1);
+
+    const set1 = new Set(["a", "b", "c"]);
+    const set2 = new Set(["b", "c"]);
+    expect(set1.isSubsetOf(set2)).toBeFalse();
+    expect(set2.isSubsetOf(set1)).toBeTrue();
+});