Datumizes DNA blocks, makes DNA cleaner in general (#92061)

Moves all the dna block handling onto singleton datums initialized
inside global lists, to make the handling dna less of a copy-paste mess
and make adding new blocks significantly easier. There is still some
work to be done in the copypaste department but ultimately that falls
under its own PR scope after the core refactor goes through. (Ill
probably do those but it will also be easier for everyone else as the
code is now significantly less of an eyesore)

Both features and identities have been tested through and through, and
seem to be working fine.

Also removed the reliance on weird hardcoded lookup tables for length,
and other similar things that just didn't make sense when I was passing
through DNA code. There's a lot more that fall out of scope for this
exact PR's goal however

I've been told the maintainers will love me for doing this

🆑
code: feature keys are no longer magical strings floating around the
codebase and use proper defines
refactor: DNA blocks are now handled with singleton datums.
/🆑

code/modules/admin/verbs/secrets.dm

@@ -578,7 +578,7 @@ ADMIN_VERB(secrets, R_NONE, "Secrets", "Abuse harder than you ever have before w
 				SEND_SOUND(H, sound(SSstation.announcer.event_sounds[ANNOUNCER_ANIMES]))
 
 				if(H.dna.species.id == SPECIES_HUMAN)
-					if(H.dna.features["tail_human"] == "None" || H.dna.features["ears"] == "None")
+					if(H.dna.features[FEATURE_TAIL] == "None" || H.dna.features[FEATURE_EARS] == "None")
 						var/obj/item/organ/ears/cat/ears = new
 						var/obj/item/organ/tail/cat/tail = new
 						ears.Insert(H, movement_flags = DELETE_IF_REPLACED)