Singletons and abstract_types! (#4760)

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## About The Pull Request
I finally have the pleasure of killing decl by giving it a new name and
updated handling.
Also abstract_types now have universal support!

Based on:
Baystation12/Baystation12/pull/32114
Baystation12/Baystation12/pull/32687

## Changelog
🆑
code: /decl/ is now /singleton/
code: abstract_type is now on the /datum/ level
/🆑

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code/modules/lore_hardcoded/_hardcoded.dm

@@ -1,9 +1,10 @@
 /datum/lore
 	/// abstract type
-	var/abstract_type = /datum/lore
+	abstract_type = /datum/lore
 
 /datum/lore/character_background
 	abstract_type = /datum/lore/character_background
+
 	/// name
 	var/name = "Unknown"
 	/// id - **must be unique on subtypes