Add support for ABNF

Augmented Backus–Naur form ([ABNF][]) is a metalanguage used to specify
language grammars.

[ABNF]: https://en.wikipedia.org/wiki/Augmented_Backus–Naur_form

lib/linguist/languages.yml

@@ -49,6 +49,13 @@ ABAP:
   - ".abap"
   ace_mode: abap
   language_id: 1
+ABNF:
+  type: data
+  ace_mode: text
+  extensions:
+  - ".abnf"
+  tm_scope: source.abnf
+  language_id: 429
 AGS Script:
   type: programming
   color: "#B9D9FF"