Build 0.5

This week, I've added some assets and restructured a lot of the code. 

Currently, there's a problem with LineRenderers in WebGL builds. The lines show up in the Scene view and the Editor view, but don't show up in the build. I'll be working on a workaround for next week - although I may not need one when I replace the lines with actual assets.

I've added some sort of animal for the numbers 1-5 and the three operators +, - and x. I want to see if people will think the animations are too distracting. In addition, I transitioned the application to URP and made a nice animation for showing operation order.

I've also tried my hand at making a tutorial. I'm having a bit of trouble making the animations and thinking of a proper way to get across the instructions in a easy-to-understand way, but I'm trying to prioritize using symbols and/or pictures to accomplish this instead of text. Unfortunately I literally cannot convert some words to pictures so I'll keep working on it as the project continues.