Add the Dargin (Dargwa) language (dar)

Requested at
https://translatewiki.net/wiki/Thread:Support/Dargin_language

Autonym according to:
Х.ГӀ. Юсупов, «Даргала-урусла словарь», ХӀ. ЦӀадасала уличилси
Мезла, литературала ва искусствола институт, МяхIячкъала 2014
and verified with speakers.

data/langdb.yaml

@@ -149,6 +149,7 @@ languages:
   cy: [Latn, [EU], Cymraeg]
   da: [Latn, [EU], dansk]
   dag: [Latn, [AF], dagbanli]
+  dar: [Cyrl, [EU], дарган]
   de-at: [Latn, [EU], Österreichisches Deutsch]
   de-ch: [Latn, [EU], Schweizer Hochdeutsch]
   de-formal: [Latn, [EU], Deutsch (Sie-Form)]