Objective

• Reduce compilation time

Solution

• Make spirv and glsl shader format support optional. They are not needed for Bevy shaders.
• on my mac (where shaders are compiled to msl), this reduces the total build time by 2 to 5 seconds, improvement should be even better with less cores

There is a big reduction in compile time for naga, and small improvements on wgpu and bevy_render

This PR with optional shader formats enabled timings:

This PR:

Migration Guide

• If you want to use shaders in spirv, enable the shader_format_spirv feature
• If you want to use shaders in glsl, enable the shader_format_glsl feature