LibJS: Implement the "instanceof" operator

This operator walks the prototype chain of the RHS value and looks for
a "prototype" property with the same value as the prototype of the LHS.

This is pretty cool. :^)

Libraries/LibJS/Tests/instanceof-basic.js

@@ -0,0 +1,7 @@
+function Foo() {
+    this.x = 123;
+}
+
+var foo = new Foo();
+if (foo instanceof Foo)
+    console.log("PASS");