Generate/prepare completions manually

Description:

As mentioned in this GitHub comments, https://github.com/dandavison/delta/issues/1022#issuecomment-1232363746, completions from delta repo is outdated (might not gonna updated). Instead, git-delta now have command to generate shell completions (https://github.com/dandavison/delta/pull/1561). The command to generate completion is delta --generate-completion=zsh. Completion option is zsh, bash, fish, elvish, and powershell.

Additional info:

• package version(s): 0.18.2-4 completion command first appear in v0.17.0
• config and/or log files: -
• link to upstream bug report, if any: https://github.com/dandavison/delta/issues/1022#issuecomment-1232363746

Steps to reproduce:

1. Install git-delta
2. git-delta have outdated completions from git-delta repo