build: Initial support for building demo (shareware) iwad

[GeorgePieVG]

Current status:

• Removed all lumps from the full version (and added new ones from shareware like ORDER)
• Removed all graphics from the full version
• Removed all maps from the full version
• Adapted all the maps from the first episode to not include enemies from later ones - TODO. I would also prefer to make the first episode vanilla compatible.
• Removed all sprites from the full version - TODO
• Removed all sounds from the full version - TODO
• Removed all textures and flats from the full version - IMHO, this is unnecessary and difficult to do, considering how small the texture set from the original shareware heretic is.