This patch adds a basic Zsh completion script for the commands and
targets provided by Meta/serenity.sh. There's some room for improvement
here, e.g. we could provide completion for available CMake targets -
currently completion stops after serenity.sh <command> <target>.
You can enable it by adding this to your .zshrc before completions are
loaded:
fpath=($SERENITY_SOURCE_DIR/Meta/ShellCompletions/zsh $fpath)
