Add lang option to vulkano_shaders::shader! macro for source language selection

[nshyrei]

Adds source_language field to MacroInput to support compiling shaders in multiple languages (e.g. HLSL, Slang), motivated by #2622. Changes adapted from #2568 as suggested by @marc0246.

Currently adds HLSL as the first supported language variant, which is passed to shaderc during shader compilation.

Closes #2568

[CptPotato]

Side note: In order to support bindless for HLSL we'd need an equivalent of vulkano.glsl. I think this can be tackled separately, though.

[nshyrei]

@CptPotato I can remove the call to shaderc with HLSL if it produces unsupported features.

[marc0246]

No no, it's fine. vulkano.hlsl should be a separate concern, not one for this PR.

[marc0246]

You need to run cargo +nightly fmt.

[nshyrei]

@marc0246 done.

[marc0246]

Thank you for the PR ❤️ Next, you can add the slang lang option. You could use libslang like we use libshaderc, but I think it's best to move away from this. We don't need to link against anything because this is a proc-macro... It doesn't end up in the binary. And a program invocation is much easier to work with and less set up for the user.

[nshyrei]

@marc0246 You mean you want to call libshaderc and libslang as a subprocess in Rust directly instead of using a shaderc crate (and some equivalent for slang) ?

[marc0246]

I mean not using libshaderc or libslang. Because those are libraries that we need to link against, which is more set up for the user, and not necessary because it doesn't end up in the binary anyway. Instead, I would like us to use Command::new("glslc") / Command::new("slangc").