`cargo gpu remove`

• cargo gpu remove --shader-crate ../shader-crate-template: should remove the rustc_codegen_spirv install of said shader crate
• cargo gpu remove --all should competely remove the entire cache dir, which would cleanup old installations by old cargo-gpu versions as well

Open Questions:

• remove or clean?
• cargo gpu remove <rust_gpu_version> We don't have a standardized way to declare a shader crate or a rust-gpu version. This would also be nice to have for installing
• Should we prompt the user about uninstalling toolchains as well? Note that two installs can use the same toolchain version. We could check every other install on whether the toolchain is used and prompt the user if noone is using it, which could be made easier by having some metadata file per install with the toolchain cached in it (among other things).

Start at the cargo gpu command implementation and trace how shader-crate installations and the cache directory are recorded. Define the removal behavior for a named shader crate and for --all, including whether unused toolchains are removed. Done means both cleanup modes work without affecting installations or shared toolchains.