Add `cargo gpu show targets`

[schell]

From #34 - add "show" command that lists the available spirv targets.

[alphastrata]

I'd like to take a stab at this one (if nobody else has claimed it), because it's been bothering me today in my quest to have a template repo for rust-gpu projects for future use...

Maybe one of the maintainers or @schell could validate whether or not we'd be happy with something simple like calling the spirv-tools ?
i.e just whip something up that grabs the output of a call to spirv-val ?

❯ spirv-val --version
SPIRV-Tools v2022.2-dev unknown hash, 2022-02-16T16:37:15
Targets:
  SPIR-V 1.0
  SPIR-V 1.1
  SPIR-V 1.2
  SPIR-V 1.3
  SPIR-V 1.4
  SPIR-V 1.5
  SPIR-V 1.6
  SPIR-V 1.2 (under OpenCL 2.2 Full Profile semantics)
  SPIR-V 1.0 (under Vulkan 1.0 semantics)
  SPIR-V 1.3 (under Vulkan 1.1 semantics)
 SPIR-V 1.4 (under Vulkan 1.1 semantics)
 SPIR-V 1.5 (under Vulkan 1.2 semantics)
 SPIR-V 1.6 (under Vulkan 1.3 semantics)

then we take the Vulkan ones?

Or, do we need to look at the spirv-tools-rs wrapper crate?'s https://docs.rs/spirv-tools/latest/spirv_tools/enum.TargetEnv.html kinda thing?