Add support for completion

[remyleone]

I think it is possible to add support for native completion using:

generate_completions_from_executable(bin/"pack", "completion", "script", shell_parameter_format: :none)

By doing so, the completion will be already included natively on macos

Pack:
  Version:  0.29.0+git-95c8060.build-4209
  OS/Arch:  darwin/amd64

Default Lifecycle Version:  0.15.2

Supported Platform APIs:  0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10