Meta: Add basic Zsh completions for serenity.sh

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)

.gitignore

@@ -4,6 +4,7 @@
 !Makefile
 !LICENSE
 !Base/**
+!Meta/ShellCompletions/**
 
 *.o
 *.ao