Implement enum attributes

[kennethloeffler]

This PR closes #383 by by adding a new tagged enum variant, EnumItem, that contains the name of the EnumItem as well as its value, and implementing the attributes reader/writer against the specification at rojo.dom.space.

Additionally, it adds a codec to rbx_dom_lua to handle the new EnumItem variant.

It also updates some docs, since the link that's currently there doesn't work anymore.

I went back and forth on what the fields should be named in the serde representation, and settled on type and value. This more or less mirrors EnumItem.EnumType and EnumItem.Value on Roblox - but maybe type should be enumType instead?

[Dekkonot]

This is potentially annoying for upstream Rojo users (they'll have to use EnumItem instead of Enum for attributes) but it's probably fine?

Do want to implement serialization for this type when it's used as a property so that consumers don't have to be careful?

[kennethloeffler]

This is potentially annoying for upstream Rojo users (they'll have to use EnumItem instead of Enum for attributes) but it's probably fine?

Yeah, it's a bit annoying, but this isn't particularly common use case, the shape of the data is different, and Rojo users only have to write the explicit syntax for enum properties very rarely, so confusion between EnumItem and Enum should be kept to a minimum

[Dekkonot]

Realized immediately after I hit approve that this needs a changelog entry... 😅

[kennethloeffler]

Is just the rbx_dom_weak changelog okay?

[Dekkonot]

...I was more thinking an rbx_types changelog entry.

[kennethloeffler]

Oh god, I'm losing the plot!