pkgctl: introduce version check subcommand

When a .nvchecker.toml file exists use nvchecker to compare the current pkgver with the latest release.

Example:

[jelle@t14s][~/projects/packages/step-cli]%cat -p .nvchecker.toml
[step-cli]
source = "github"
github = "smallstep/cli"
use_max_tag = true
prefix = "v"

[jelle@t14s][~/projects/packages/step-cli]%~/projects/devtools/test/bin/pkgctl version check
  -> New version 0.25.0 is available upstream