Use spritesheets instead of individual image files

[SolsticeSpectrum]

adryd's port of neko to javascript is one of the best implementation of the silly cat
https://github.com/adryd325/oneko.js

I am currently working on some conversion tools to get more animals from these repos!
https://github.com/tie/oneko/tree/master/bitmaps
https://github.com/eliot-akira/neko/tree/main/2023-icon-library

But the tools I am making generate spritesheets with placements in exact same positions
as in adryd's oneko.gif so they don't have to change the mapping in the code

Thus if the values were copied to this version, it will be able to load the spritesheets I am working on
getting closer to the idea of having multiple fur colors that you pitched in #23
And not only fur colors but different animals all together
plus it will make the assets folder less chaotic

Also when at it, I think some of the move
ment/animation logic could be borrowed too, for example this version makes the cat
fall asleep very fast and also if close to the mouse, the cat twitches like crazy, also diagonal movement is faster which is not
an issue in the javascript version

Overall I think checking out that javascript version could be beneficial and fix the issues with movement and animations

Here is an example spritesheet for testing if you need