Lua: Allow access to skin.sprites[]

This exposes skin.sprites[] for usage in scripts, for accessing a skins sprite set, only numframes is exposed so far though, as it's to my understanding there's already a function for what spriteframes deals with.

You can get the number of frames a sprite has by doing something like this for example, skins["sonic"].sprites[spr2].numframes, if numframes returns 0 that means spr2 does not exist in the sprite set for the specified skin. You can also get a super version by doing FF_SP2SUPER|spr2.

#skins["sonic"].sprites to get the number of player sprites.