levels/world2: Add "The Dark Tunnel"

"The Dark Tunnel" is a Ghost Forest level featuring a tunnel with
diverse Ghost Forest enemies, and some secret areas.

Ref: #58 ("levels/world2: Add getting_hot.stl +
dark_tunnel.stl")
[[email protected]: fixed level location, and extended commit message]
Reviewed-by: Max Teufel <[email protected]>
Tested-by: Max Teufel <[email protected]>
Cc: Carsten Wirtz <[email protected]>
Cc: Max Teufel <[email protected]>
Cc: alex jockers <[email protected]>