Add newtype for launch::Slice path globs

[Malax]

As per spec, these are Go standard library globs and must follow that syntax (https://pkg.go.dev/path/filepath#Match). This is potentially more complex to implement as a newtype than the ones we currently have.

[edmorley]

I'd be inclined to wontfix this to be honest (if we're going to have to reimplement Go path globs). Usage of slice layers is likely going to be pretty rare, and people using it can just read the docs. Whilst it's nice that libcnb.rs can prevent a subset of errors at runtime though typing and nice DX, I don't think it's viable for it to prevent every class of issue.