vim-patch:9.1.1173: filetype: ABNF files are not detected (#32751)

Problem:  filetype: ABNF files are not detected
Solution: detect '.abnf' file as abnf filetype and
          include an abnf syntax plugin (A4-Tacks).

References:
- RFC5234
- RFC7405

closes: vim/vim#16802

9f827ec587

Co-authored-by: A4-Tacks <wdsjxhno1001@163.com>

runtime/lua/vim/filetype.lua

@@ -198,6 +198,7 @@ local extension = {
   abap = 'abap',
   abc = 'abc',
   abl = 'abel',
+  abnf = 'abnf',
   wrm = 'acedb',
   ads = 'ada',
   ada = 'ada',