391: Change `include_spirv` macro to accept tokens r=cwfitzgerald a=neachdainn
The `include_spirv` macros was set to accept literals only. Unfortunately, this means that other macros, such as `concat` and `env` aren't accepted, making it unusable with build script-compiled shaders. This change allows those macros to be used to load the SpirV at the expense of a potentially less helpful error message as the `literal` requirement is differed to the `include_bytes` invocation.
Co-authored-by: Nathan Kent <nkent2@cs.rochester.edu>