Add extpack subcommand completion

[x] This compdef is not already available in zsh.
[x] This compdef is not already available in their original project.
[ ] I am the original author, or I have authorization to submit this work.
[x] This is a finished work.
[x] It has a header containing authors, status and origin of the script.
[x] It has a license header or I accept that it will be licensed under the terms of the Zsh license.

zsh-users / zsh-completions