patch 9.1.1173: filetype: ABNF files are not detected

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: #16802

Signed-off-by: A4-Tacks <wdsjxhno1001@163.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/abnf.vim

@@ -0,0 +1,33 @@
+" Vim compiler file
+" Language:	abnf
+" Maintainer:	A4-Tacks <wdsjxhno1001@163.com>
+" Last Change:	2025 Mar 05
+" Upstream:	https://github.com/A4-Tacks/abnf.vim
+
+" Implementing RFC-5234, RFC-7405
+
+if exists('b:current_syntax')
+  finish
+endif
+
+syn case ignore
+
+syn match  abnfError	/[<>"]/
+syn match  abnfComment	/;.*/
+syn match  abnfOption	/[[/\]]/
+syn region abnfString	start=/\(%[si]\)\="/ end=/"/ oneline
+syn region abnfProse	start=/</ end=/>/ oneline
+syn match  abnfNumVal	/\v\%b[01]+%(%(\.[01]+)+|-[01]+)=>/
+syn match  abnfNumVal	/\v\%d\d+%(%(\.\d+)+|-\d+)=>/
+syn match  abnfNumVal	/\v\%x[0-9a-f]+%(%(\.[0-9a-f]+)+|-[0-9a-f]+)=>/
+syn match  abnfRepeat	/\v%(%(<\d+)=\*\d*|<\d+ =)\ze[^ \t\r\n0-9*/)\]]/
+
+hi def link abnfError		Error
+hi def link abnfComment		Comment
+hi def link abnfOption		PreProc
+hi def link abnfString		String
+hi def link abnfProse		String
+hi def link abnfNumVal		Number
+hi def link abnfRepeat		Repeat
+
+" vim:noet:ts=8:sts=8:nowrap