runtime(groff,nroff): improve ftplugin

- set options in ftplugin but not in syntax
- implement ftplugin/groff.vim (wrapper of ftplugin/nroff.vim)

closes: #17174

Signed-off-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/groff.vim

@@ -0,0 +1,15 @@
+" Vim syntax file
+" Language:  groff(7)
+" Maintainer: Eisuke Kawashima ( e.kawaschima+vim AT gmail.com )
+" Last Change: 2025 Apr 24
+
+if exists('b:did_ftplugin')
+  finish
+endif
+
+let b:nroff_is_groff = 1
+
+runtime! ftplugin/nroff.vim
+
+let b:undo_ftplugin .= '| unlet! b:nroff_is_groff'
+let b:did_ftplugin = 1