[snaplet] create completion spec

[gusfune]

Sanity checks

• [X] My issue relates to a specific CLI completion spec (e.g. git checkout is missing options in git completion spec). If your issue is more general, please create your issue here: withfig/fig
• [X] I have searched github.com/withfig/autocomplete/issues and there are no duplicates of my issue
• [X] I would like to work on this.

What CLI tool does this relate to?

snaplet

Which statement makes the most sense?

This completion spec does not exist

Issue Details

Snaplet is a developer tool for snapshotting and changing production values for development.