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>
