LibJS: Add StringPrototype and make it the prototype of StringObject

This patch adds String.prototype.charAt() to demonstrate that prototype
property lookup works, and that you can call a prototype function on an
object, and it will do what you expect. :^)

Libraries/LibJS/StringObject.h

@@ -37,8 +37,12 @@ public:
 
     virtual void visit_children(Visitor&) override;
 
+    const PrimitiveString* primitive_string() const { return m_string; }
+
 private:
     virtual const char* class_name() const override { return "StringObject"; }
+    virtual bool is_string_object() const override { return true; }
+
     PrimitiveString* m_string { nullptr };
 };