Add winit/wgpu backends

[lukexor]

Currently sdl2 is the only available backend, which has served this project well but it carries a lot of baggage and dependency cost and creates a divide when it comes to web support.

winit/wgpu provides better cross-platform support for metal, vulkan, openGL, direct3d, webgl, and webgpu.

I'm not sure if I want to keep supporting sdl2 as an option via feature flags or not, given the maintenance burden but the goal should be minimal breaking API changes. It's convenient, then, that winit seems to have adopted a very similar trait-based APi.

This change will likely also necessitate adding in crates like cpal for audio and gilrs for cross platform gamepad support.