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/NumberObject.h

@@ -31,6 +31,8 @@
 namespace JS {
 
 class NumberObject : public Object {
+    JS_OBJECT(NumberObject, Object);
+
 public:
     static NumberObject* create(GlobalObject&, double);
 
@@ -41,8 +43,6 @@ public:
     virtual Value value_of() const override { return Value(m_value); }
 
 private:
-    virtual const char* class_name() const override { return "NumberObject"; }
-
     double m_value { 0 };
 };