exercism: install shell completions

Exercism ships both with pre-generated completions [0], and has
`completion` command to generate completions for
bash/zsh/fish/powershell.
Initially I was planning to use the latter, but seems like due to
a bug (?) in cobra-generated completions (probably [1]), the resulting
completions aren't fully working (if at all);
so this makes use of the ones provided in the repo. I'll probably update
this once more to use auto-generated completions once I manage to update
cobra dependency for exercism CLI itself.

[0] https://github.com/exercism/cli/tree/main/shell
[1] https://github.com/spf13/cobra/pull/1940

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