LibJS: Add Array.prototype.join()

And share the code with Array.prototype.toString() :^)

Libraries/LibJS/Tests/Array.prototype.join.js

@@ -0,0 +1,13 @@
+load("test-common.js");
+
+try {
+    assert(Array.prototype.push.length === 1);
+
+    assert(["hello", "friends"].join() === "hello,friends");
+    assert(["hello", "friends"].join(" ") === "hello friends");
+    assert(Array(3).join() === ",,");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}