add "switch" snippet to js-mode (#217)

Inspired by other language modes (go, cc, php, etc.).

snippets/js-mode/switch

@@ -0,0 +1,10 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: switch
+# key: sw
+# --
+switch (${1:condition}) {
+  case ${2:expression}:
+    $0
+    break;
+  default:
+}