[bug#30114] Add fish completion to Guix, remove fish-guix

[ng0]

If you require a formal statement somewhere that I relicense fish-guix to be
able to be merged into Guix, let me know. Otherwise I'll just make a commit
in the old repository mentioning it.

These two patches: Add etc/completion/fish/guix.fish, add the file to
the Makefile + configure.ac, and remove fish-guix.

I haven't had the chance to test wether the vendor path works, it did
throw no errors when I ran make in the repository and the path resembles
what I used in fish-guix.
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0001-etc-Add-completions-for-fish.patch
Description: Text document

0002-gnu-fish-guix-Remove-it.patch
Description: Text document

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