st/box-layout: Deprecate :pack-start property
A property for reversing the visible order of children is a bit odd.
It has also been unused by actual gnome-shell code since 2010, and the somewhat related pack_start()/pack_end() API in GtkBox(Layout) is gone in GTK4.
With that in mind, turn the property into a no-op and deprecate it, so that it can be dropped next cycle.
See mutter!2206 (merged) for the mutter counterpart.