Unison support (#12715)

diff --git a/runtime/ftplugin/unison.vim b/runtime/ftplugin/unison.vim
new file mode 100644
index 0000000..76dbaef
--- /dev/null
+++ b/runtime/ftplugin/unison.vim
@@ -0,0 +1,14 @@
+" Vim filetype plugin file
+" Language:             unison
+" Maintainer:           Anton Parkhomenko <anton@chuwy.me>
+" Latest Revision:      2023-08-07
+
+if exists("b:did_ftplugin")
+  finish
+endif
+let b:did_ftplugin = 1
+
+let b:undo_ftplugin = "setl cms< isk<"
+
+setlocal commentstring=--\ %s
+setlocal iskeyword+=!,'