Singletons + refactor of /datum/observ (#15487)

code/core/datum/IsAbstract.dm

@@ -0,0 +1,23 @@
+/**
+* Abstract-ness is a meta-property of a class that is used to indicate
+* that the class is intended to be used as a base class for others, and
+* should not (or cannot) be instantiated.
+* We have no such language concept in DM, and so we provide a datum member
+* that can be used to hint at abstractness for circumstances where we would
+* like that to be the case, such as base behavior providers.
+*/
+
+/// If set, a path at/above this one that expects not to be instantiated.
+/datum/var/abstract_type
+
+/// If true, this datum is an instance of an abstract type. Oops.
+/datum/proc/IsAbstract()
+	SHOULD_NOT_OVERRIDE(TRUE)
+	return type == abstract_type
+
+/// Passed a path or instance, returns whether it is abstract. Otherwise null.
+/proc/is_abstract(datum/thing)
+	if (ispath(thing))
+		return thing == initial(thing.abstract_type)
+	if (istype(thing))
+		return thing.IsAbstract()