LibJS+LibWeb: Add JS::Object::inherits(class_name)

To allow implementing the DOM class hierarchy in JS bindings, this
patch adds an inherits() function that can be used to ask an Object
if it inherits from a specific C++ class (by name).

The necessary overrides are baked into each Object subclass by the
new JS_OBJECT macro, which works similarly to C_OBJECT in LibCore.

Thanks to @Dexesttp for suggesting this approach. :^)

Libraries/LibJS/Runtime/BooleanObject.h

@@ -30,6 +30,8 @@
 
 namespace JS {
 class BooleanObject : public Object {
+    JS_OBJECT(BooleanObject, Object);
+
 public:
     static BooleanObject* create(GlobalObject&, bool);
 
@@ -42,7 +44,6 @@ public:
     }
 
 private:
-    virtual const char* class_name() const override { return "BooleanObject"; }
     virtual bool is_boolean_object() const override { return true; }
     bool m_value { false };
 };