Modernize and fix

[Kaydax]

Like the last PR but actually tested and done right.

• Bumps the entire project to be .net standard 2.1 targeting CS 10.0
• Fixes all warnings
• Disable nullables / implicate usings
• Converts Vector2 to use System.Numerics (IDK why it wasn't)
• Update test to .net 7
• Add VS Code OSS launch config (using netcoredbg)
• Add editor config (The code has been reformatted to conform to this, but this can be reverted if needed)

[ameoverflow]

Oh, I didn't see your PR until now, kinda forgot about this fork. I'll accept your PR and create 3.4.1

[ameoverflow]

As for code not using System.Numerics that was initially because I thought that Unity didn't have System.Numerics and I meant to remove it and replace it like you did so yeah. Also I'm not happy that you've changed indentation of the whole project from 4 spaces to 2 so I've reverted it to 4