add varlink syntax

Varlink is an interface description format and protocol that aims
to make services accessible to both humans and machines in the
simplest feasible way.

See: https://varlink.org/ and https://github.com/varlink/ for
more information

.gitmodules

@@ -120,3 +120,6 @@
 [submodule "assets/syntaxes/Jsonnet"]
 	path = assets/syntaxes/Jsonnet
 	url = https://github.com/gburiola/sublime-jsonnet-syntax.git
+[submodule "assets/syntaxes/varlink"]
+	path = assets/syntaxes/varlink
+	url = https://github.com/varlink/syntax-highlight-varlink.git