Finish WORLD.md (the docs for the "json/areas" files)

[Synival]

WORLD.md should contain all information necessary to write entire areas from scratch. This means:

1. Areas
2. Rooms
   1. Exits
   2. Resets
3. Objects
   1. All object types
4. Mobs
5. (Probably not) Mob programs

Each section needs the following documented:

1. Required properties
2. Optional properties
3. When applicable: Flag / type values

A few extra sections:

1. "Extended" human-readable multiline format
2. Files in json/config
3. What are "portals"?
4. Help files
5. The files in json/meta