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/instruments/instrument_data/_instrument_data.dm

@@ -21,14 +21,15 @@
 			. += I.id
 
 /datum/instrument
+	/// Used for categorization subtypes.
+	abstract_type = /datum/instrument
+
 	/// Name of the instrument
 	var/name = "Generic instrument"
 	/// Uniquely identifies this instrument so runtime changes are possible as opposed to paths. If this is unset, things will use path instead.
 	var/id
 	/// Category
 	var/category = "Unsorted"
-	/// Used for categorization subtypes
-	var/abstract_type = /datum/instrument
 	/// Write here however many samples, follow this syntax: "%note num%"='%sample file%' eg. "27"='synthesizer/e2.ogg'. Key must never be lower than 0 and higher than 127
 	var/list/real_samples
 	/// assoc list key = /datum/instrument_key. do not fill this yourself!