runtime(abnf): include ABNF filetype plugin

closes: #17239

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

runtime/ftplugin/abnf.vim

@@ -0,0 +1,17 @@
+" Language:	abnf
+" Maintainer:	A4-Tacks <wdsjxhno1001@163.com>
+" Last Change:	2025-05-02
+" Upstream:	https://github.com/A4-Tacks/abnf.vim
+
+if exists('b:did_ftplugin')
+  finish
+endif
+let b:did_ftplugin = 1
+
+let b:undo_ftplugin = 'setlocal iskeyword< comments< commentstring<'
+
+setlocal iskeyword=@,48-57,_,-,192-255
+setlocal comments=:;;,:;
+setlocal commentstring=;%s
+
+" vim:ts=8