Expose `clap`'s shell completion generation methods via CLI

[ed9w2in6]

Who, What, and Why

As a user,
I want shell completion,
so that I do not need to memorise the CLI or rely on the CLI's help utility.
--> This allows a smoother CLI usage experience and potentially further improves CLI discoverability.

Specific scenarios:

• subcommand completion
  • there are a lot of subcommands, will be great to be able to search through them and list them all out temporarily with shell completions
• option completion
  • same reasoning as above
• argument completion
  • subcommands such as spotify_player get key <key> have arguments with specific values. Since they are nicely named but long, completion can speed this up by reducing risk of typo.

shell completion: custom TAB-completion available for some shells that requires custom script provided by the CLI program
some shells: clap_complete 4.4.4 provides bash, elvish, fish, powershell and zsh

How (roughly)

new dependencies

• clap_complete: https://docs.rs/clap_complete/latest/clap_complete/index.html
  • clap = "4.4.10" requires clap_complete, our master branch is at clap = "4.4.10" as of [2023-12-04]

implementation directions

1. provide subcommand that prints to stdout
   • The documentation above included an example implementation that prints completion script to stdout.
2. generate at build time
   • The example for generate_to included an implementation that generates completions within the build.rs script during build time: https://docs.rs/clap_complete/latest/clap_complete/generator/fn.generate_to.html

When

NOT urgent.
The CLI is already very much usable at the current state.

Other informations

• spotify-tui uses implementation direction 1, but they are using clap = "2.33.3":
  https://github.com/Rigellute/spotify-tui/blob/c4dcf6b9fd8318017acbdd7ec005955e26cf2794/src/main.rs#L171-L185
• There was an issue with the said feature for clap = "2.33.3" BUT it seems to be fixed since (clap = "4.3.11", clap_complete = "4.3.2"): zsh: optional flag with takes_value breaks completions clap-rs/clap#1822