Add fish shell completions

[cljoly]

Generate fish shell completions at build time. Empower fish users with
completions.

Completions are copied in the recommended directory.

Describe what this PR changes. Why this is wanted. And, if needed, how it does it.

Git checklist:

• Describe the change in CHANGELOG.md under the [Unreleased] header.
• Check that commits follow the Mullvad coding guidelines

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This change is 

[faern]

The code changes look good! Thank you for this contribution.

I have some nitpicks about the changelog that I would like you to fix.

The git history is also currently in a form we can't merge. We do not merge master into feature branches and then back. Please rebase the feature branch on latest master and squash commits in such a way that the feature branch has no merge commits. This small feature can preferably just be a single commit.

[cljoly]

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CHANGELOG.md, line 28 at r2 (raw file):

Previously, faern (Linus Färnstrand) wrote…

My bad! It's for two platforms. So keeping it under the generic header is fine. It could be extended to say "on Linux and macOS" but that's just nitpick. I don't think fish exists for Windows anyway? Resolved.

Yes, no windows support for fish: fish-shell/fish-shell#319

[cljoly]

Thanks for your review. I think I have addressed your comments.

[faern]

Thank you very much. I will merge this asap.

[cljoly]

Awesome!