Feature request: Add the shell completions to make install

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Issue description

Shell completions aren't installed by dunst's Makefile, hence aren't packaged by distribution policy to provide vanilla packages. Adding them, even gated by a flag, would legitimate packaging them.

Installation info

• Version: Dunst - A customizable and lightweight notification-daemon 1.9.2 (2023-04-20)
• Install type: extra/dunst 1.9.2-1
• Window manager / Desktop environment: i3
• Distro: Arch Linux

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Thanks!