LibJS: Add Object.{isExtensible,preventExtensions}()

Author: https://github.com/mattco98
Commit: d5ae73a63b
Pull-request: https://github.com/SerenityOS/serenity/pull/2474

Libraries/LibJS/Tests/Object.isExtensible.js

@@ -0,0 +1,22 @@
+load("test-common.js");
+
+try {
+    assert(Object.isExtensible() === false);
+    assert(Object.isExtensible(undefined) === false);
+    assert(Object.isExtensible(null) === false);
+    assert(Object.isExtensible(true) === false);
+    assert(Object.isExtensible(6) === false);
+    assert(Object.isExtensible("test") === false);
+
+    let s = Symbol();
+    assert(Object.isExtensible(s) === false);
+
+    let o = { foo: "foo" };
+    assert(Object.isExtensible(o) === true);
+    Object.preventExtensions(o);
+    assert(Object.isExtensible(o) === false);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}