Add zsh-completions

The [zsh-completions](https://github.com/zsh-users/zsh-completions) repo
contains completions for various commands but not all of these are
useful so the `update-completion-links.zsh` script inspects the `PATH`
to determine which of the completions should be symlinked to
`~/.local/share/zsh/site-functions` which resides on the `fpath`.

.conduit.yaml

@@ -29,10 +29,13 @@
     - https://github.com/zsh-users/zsh-autosuggestions.git
     - https://github.com/zsh-users/zsh-history-substring-search.git
     - https://github.com/zsh-users/zsh-syntax-highlighting.git
+    - https://github.com/zsh-users/zsh-completions.git
     - https://github.com/junegunn/fzf.git
 - command:
     - fzf/install --bin
 - symlink:
     - {src: fzf/bin/fzf, dst: ~/.local/bin/fzf}
     - {src: fzf/bin/fzf-tmux, dst: ~/.local/bin/fzf-tmux}
+- command:
+    - zsh update-completion-links.zsh
 - message: zsh will be the default prompt after next login