core: rename `world` to `multiworld`

[alwaysintreble]

discussed in the discord. renaming it to help prevent confusion for new devs. oot and smz3 broken currently

[alwaysintreble]

took a minor stint where i was attempting to do python in Rider and it somehow completely butchered world api so that's fun.

[el-u]

Missed usages of world (usually in method signatures or local variables of type MultiWorld):

AutoWorld.py:

• call_single
• call_all
• call_stage
• World.__init__
• World.create_item.__doc__

BaseClasses.py:

• Region.__init__
• Entrance.__str__
• Location.__str__
• Spoiler.__init__

Main.py:

• main.world
• create_playthrough