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/Error.cpp

@@ -57,7 +57,6 @@ Error::~Error()
     {                                                                                                                     \
     }                                                                                                                     \
     ClassName::~ClassName() { }                                                                                           \
-    const char* ClassName::class_name() const { return #ClassName; }
 
 JS_ENUMERATE_ERROR_SUBCLASSES
 #undef __JS_ENUMERATE