patch 9.1.2110: filetype: skhd files are not recognized

Problem:  filetype: skhd files are not recognized
Solution: Detect .skhdrc and skhdrc as skhd filetype,
          include a syntax and filetype plugin, add syntax tests
          (Kiyoon Kim)

Add syntax highlighting for skhd (simple hotkey daemon for macOS)
configuration files. Includes filetype detection for skhdrc and
.skhdrc files.

Reference:
- https://github.com/asmvik/skhd

closes: #19235

Signed-off-by: Kiyoon Kim <kiyoon@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/skhd.vim

@@ -0,0 +1,20 @@
+" Vim filetype plugin file
+" Language:	skhd(simple hotkey daemon for macOS) configuration file
+" Maintainer:	Kiyoon Kim <https://github.com/kiyoon>
+" Last Change:	2026 Jan 23
+
+if exists("b:did_ftplugin")
+  finish
+endif
+let b:did_ftplugin = 1
+
+let s:cpo_save = &cpo
+set cpo&vim
+
+setlocal comments=:# commentstring=#\ %s
+setlocal formatoptions-=t formatoptions+=croql
+
+let b:undo_ftplugin = "setl com< cms< fo<"
+
+let &cpo = s:cpo_save
+unlet s:cpo_save