Refactoring: stack type goes better in a separate library

[acmenezes]

Since the stack type is generic and not necessarily related to capability we could use a different package to hold those types an allow it to be imported from multiple other points or even externally since it's pretty useful for many cases. Instead of having it on internal/capabilities/types.go we could provide a different package not sure what name and what this package could hold though.

[skattoju]

#293 is a draft PR suggesting moving it to internal/types/stack.go what do you think ?