Datumize sound keys into sound_effect_datums (#91850)

## About The Pull Request
inspired by https://github.com/tgstation/tgstation/pull/85407

we initialize a global assoc list in SSsounds named sfx_datum_by_key,
with k = SFX_KEY, v = ref to the singleton sound_effect datum, then we
can call return_sfx on the referenced datum to get an sfx
## Why It's Good For The Game
makes the code a bit easier to add onto and work with
## Changelog
🆑
refactor: refactored how sfx with multiple variants are coded, report
any issues with sounds not playing!
/🆑

code/controllers/subsystem/sounds.dm

@@ -37,6 +37,7 @@ SUBSYSTEM_DEF(sounds)
 /datum/controller/subsystem/sounds/Initialize()
 	setup_available_channels()
 	find_all_available_sounds()
+	init_sound_keys()
 
 	if(!(RUST_G))
 		to_chat(world, span_boldnotice("Sounds subsystem: No rust_g detected."))
@@ -225,4 +226,10 @@ SUBSYSTEM_DEF(sounds)
 	sound_lengths[file_path] = as_num
 	return as_num
 
+/datum/controller/subsystem/sounds/proc/init_sound_keys()
+	for(var/datum/sound_effect/sfx as anything in subtypesof(/datum/sound_effect))
+		// this is for the assoc subtype
+		if(!isnull(sfx.key))
+			GLOB.sfx_datum_by_key[sfx.key] = new sfx()
+
 #undef DATUMLESS