Expose follower array and follower_t to lua

Does exactly what it says on the title.

This adds a new source file lua_followerlib.c for follower specific lua stuff. Like the skins[] table, this doesn't allow for modification, and only for reading. Useful for lua-enhanced followers to read to identify the right follower. Also adds an entry for follower_t on userdataType()... for anyone who uses that.

Example test file (displays current follower info on screen): testfollower.pk3