Implement `Shader` API Object

[tychedelia]

When users interact with our material API, they'll need to load WGSL shaders from the filesystem. We'll want to implement a Shader asset that helps them doing so. Note, in Processing4 PShader is a more general purpose entry-point to OpenGL. Because WebGPU is a bit lower-level than GL, it doesn't make as much sense to expose a full pipeline API here. We may in the future choose to add some things like reflection driven layout information for users who want to use the low-level WebGPU APIs, but let's keep it simple to start as just a handle to a shader file.