Move shared WGSL code to imported module

[djeedai]

Move most shared WGSL code into an import module vfx_common.wgsl, and
document it. This ensures the code remains in sync between the various passes
(init/update/render) without needing some error-prone copy/paste, while making
the code easier to read too.

This change moves naga and naga_oil from being dev-only dependencies to
being proper crate dependencies. This is required to use naga_oil's import
system, like bevy_render. This makes the change a breaking one.