Adds a protection that blocks marking rank datums (#95654)

code/modules/admin/view_variables/mark_datum.dm

@@ -1,6 +1,8 @@
 /client/proc/mark_datum(datum/D)
 	if(!holder)
 		return
+	if(!D.allow_mark_datum())
+		return
 	if(holder.marked_datum)
 		holder.UnregisterSignal(holder.marked_datum, COMSIG_QDELETING)
 		vv_update_display(holder.marked_datum, "marked", "")
@@ -15,3 +17,7 @@ ADMIN_VERB_ONLY_CONTEXT_MENU(mark_datum, R_NONE, "Mark Object", datum/target as
 	SIGNAL_HANDLER
 	UnregisterSignal(marked_datum, COMSIG_QDELETING)
 	marked_datum = null
+
+/// Will this datum allow itself to be marked by vv?
+/datum/proc/allow_mark_datum()
+	return TRUE