Add Magahi

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

Autonym according to Ethnologue.

data/langdb.yaml

@@ -387,6 +387,7 @@ languages:
  # Also Geor, but the incubator is in Latn
   lzz: [Latn, [EU, ME], Lazuri]
   mad: [Latn, [AS], Madhurâ]
+  mag: [Deva, [AS], मगही]
   mai: [Deva, [AS], मैथिली]
   mak: [Latn, [AS, PA], Mangkasarak]
   mak-bugi: [Bugi, [AS, PA], ᨆᨀᨔᨑ]

data/language-data.json

@@ -2497,6 +2497,13 @@
             ],
             "Madhurâ"
         ],
+        "mag": [
+            "Deva",
+            [
+                "AS"
+            ],
+            "मगही"
+        ],
         "mai": [
             "Deva",
             [
@@ -5490,6 +5497,7 @@
             "bho",
             "awa",
             "as",
+            "mag",
             "mai",
             "hne",
             "ne",