runtime(xkb): Include a simple xkb ftplugin

Problem:  There is a xkb syntax, but no filetype plugin.
Solution: Create a filetype plugin and set the comment and commentstring
          options for the xkb filetype (xkb = X keyboard extension)

closes: #19537

Signed-off-by: GX <59413576+gx089@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/xkb.vim

@@ -0,0 +1,14 @@
+" Vim filetype plugin
+" Language:    xkb (X keyboard extension)
+" Maintainer:  The Vim Project <https://github.com/vim/vim>
+" Last Change: 2026 Mar 01
+
+if exists('b:did_ftplugin')
+  finish
+endif
+let b:did_ftplugin = 1
+
+setl comments=://
+setl commentstring=//\ %s
+
+let b:undo_ftplugin = 'setl com< cms<'