Support UMAPINFO in heretic

[kraflab]

We have a request for at least a partial implementation of UMAPINFO in heretic for the QTR2 project (happening some time in 2025).

These features specifically have been requested:

• map name
• map sky texture
• midi track
• next map, regular
• next map, secret
• par time
• author
• level number
• Intertext
• enterpic / exitpic

Most of these probably just involve turning it on, since it's currently disabled for heretic. Full UMAPINFO support isn't really possible since the spec has stuff like doom boss actions in it and no defined heretic actor list etc.