Completion that supports split subcommands(plugin-add -> plugin add)

[Kurichi]

Is your feature request related to a problem? Please describe

Based on the asdf documentation, it seems that the recommended way to separate subcommands is asdf plugin add rather than asdf plugin-add. However, only the plugin-add method is stored for completion.

Describe the proposed solution

Improve the completions script to support separated subcommands like asdf plugin add.

Describe similar asdf features and why they are not sufficient

You can do the same with asdf plugin-add, but it's a method not documented and is user-unfriendly.

Describe other workarounds you've considered

If asdf plugin-add is the recommended method, it would be considered to revise the documentation.