js: add missing start_digraph() for the js backend (#12638)

vlib/v/dotgraph/dotgraph.js.v

@@ -0,0 +1,6 @@
+module dotgraph
+
+pub fn start_digraph() {
+	println('digraph G {')
+	#JS.process.on('exit', fn () { println('}') })
+}