bash-completion: cleanup and start introducing local uenv labels completion

[albestro]

DISCLAIMER: I'm not very proficient with bash, and it's my first time dealing with CLI11 and bash completion. Any suggestion is welcome! I opted for opening this PR as draft to start sharing a working (and hopefully evolving) experiment and get some early feedback on this.

I started this as an experiment (over #128 which immediately proved to be useful) to explore how bash completion can be extended for uenv, in particular targeting auto-completion for uenv images available locally (i.e. start and run).

Completion is a broad and complicated feature, and I'd like to keep this PR focused on "uenv labels completion for locally available images" as much as possible, while dealing with other extensions separately. However, current design is quite basic and some auxiliary reworkings might be needed to make this work.

TODO

• Extract bash completion script as "external" resource #128
• Distinguish positional arguments by name (e.g. image ls {uenv} is different from start {uenv}
• Explore how to deal with multiple uenvs (currently separated by comma)
• What local images should be suggested? image ls %ARCH?