LibJS: Add new PropertyDescriptor class and related abstract operations

This is an implementation of 'The Property Descriptor Specification
Type' and related abstract operations, namely:

- IsAccessorDescriptor
- IsDataDescriptor
- IsGenericDescriptor
- FromPropertyDescriptor
- ToPropertyDescriptor
- CompletePropertyDescriptor

It works with Optional<T> to enable omitting certain fields, which will
eventually replace the Attribute::Has{Getter,Setter,Configurable,
Enumerable,Writable} bit flags, which are awkward to work with - being
able to use an initializer list with any of the possible attributes is
much more convenient.

Parts of the current PropertyAttributes implementation as well as the
much simpler PropertyDescriptor struct in Object.h will eventually be
replaced with this and completely go away.

Property storage will still use the PropertyAttributes bit flags, this
is for the layers above.

Note that this is currently guarded behind an #if 0 as if conflicts with
the existing PropertyDescriptor struct, but it's known to compile and
work just fine - I simply want to have this in a separate commit, the
primary object rewrite commit will be large enough as is.

Userland/Libraries/LibJS/Forward.h

@@ -144,6 +144,8 @@ class PrimitiveString;
 class PromiseReaction;
 class PromiseReactionJob;
 class PromiseResolveThenableJob;
+class PropertyAttributes;
+class PropertyDescriptor;
 class PropertyName;
 class Reference;
 class ScopeNode;