feat: add pack --list subcommand to Ya CLI

[lpnh]

Add -l, --list flags to the ya pack command.

It prints a list of the currently installed packages as specified in the package.toml file.

Result:

ya pack --list
plugin:
	dedukun/relative-motions
	DreamMaoMao/mime
flavor:

ya pack --help
Manage packages

Usage: ya pack [OPTIONS]

Options:
  -a, --add <ADD>  Add a package
  -i, --install    Install all packages
  -l, --list       List all installed packages
  -u, --upgrade    Upgrade all packages
  -h, --help       Print help

The implementation is probably very naive. I'm open to any feedback.

[sxyazi]

LGTM, thanks!