add spirv-unknown-naga-wgsl target via naga

[Firestar99]

Requires #309 #310 #311

[LegNeato]

I think this is fine and later when we change it to have something generic like "gpu" we can document it / support the old style as well. Do you think instead it should be spirv-[unknown-]naga-wgsl or spirv-[unknown-]wgpu-wgsl? Probably not but wanted to check in case you hadn't thought of those options and they are more appropriate. I doubt we'd have another or "pure" wgsl backend for a long time.

[Firestar99]

I actually think we should add naga to the target, since we could be having a *-naga-glsl and a *-cross-glsl target via spirv-cross

[Firestar99]

It has been renamed to spirv-unknown-naga-wgsl and should be quite usable already. There's still a bunch of failing compiletests due to naga not parsing some spv structure properly, they should either be ignored or checked upon before merging. May just opt for ignoring them all and fixing them up at a later date.

Nice side effect of naming it naga-wgsl: compiletest can be ignored with ignore-naga so any future naga targets will also be ignored.