examples: add modem watcher example in JS

examples/modem-watcher-javascript/main.js

@@ -0,0 +1,11 @@
+
+const Mainloop = imports.mainloop;
+const GLib     = imports.gi.GLib;
+
+const modemWatcher = imports.modemWatcher;
+
+function start() {
+    let watcher = new modemWatcher.ModemWatcher();
+
+    Mainloop.run();
+}