Add zsh completions (compdef) for plasmashell

Bah, forgot to open an MR for this one!

This completion file shares a function with plasmoidviewer from sdk, so they will have be kept in sync. Deduplicating it would rely on an extra file being present in a system, and it's unclear which repo that file would have to be in. I'll just add a comment asking future devs to make sure they send patches to both.

Note: it uses the new variable KDE_INSTALL_ZSHAUTOCOMPLETEDIR, which was added in ECM 5.101.

Read more: frameworks/extra-cmake-modules!310 (merged)