feat(issue-1071): adds zellij completion

[dvrd]

Resolves #1071

• [x] This compdef is not already available in zsh.
• [x] This compdef is not already available in their original project.
• [x] 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.

Reasoning

Currently zellij doesn't automatically add completions, you have to add them yourself from a cli command provided by the project but is your decision how it gets sourced

Changes

• Adds completion for zellij and utility functions

[syohex]

Completions are not accepted when already available in their original project.

I'm afraid that we cannot accept its completion because zellij provides zsh completion by itself according to the above reason. We can generate it by zellij setup --generate-completion zsh