Biocubes are now proper stacks (+ shrinks their sprite)

[Absolucy]

About The Pull Request

this makes biocubes a proper subtype of /obj/item/stack, meaning they can auto stack now (max amount of 20).

also added a biocube_cost var - rn only can be changed with varediting, but in a later PR I plan to refactor and improve composters

Why It's Good For The Game

spam clicking biocubes is annoying and kinda hurts my wrist. so let's start cleaning up the code a bit, and get rid of some of spam-clicking!

also this makes the sprite less comically huge.

Changelog

🆑
refactor: Biocubes are now proper stacks, and behave as such.
image: Made the biocube sprite slightly smaller.
/:cl:

[Absolucy]

tm had no issues, and technically this is just a code improvement with QoL as a side effect, so yolo