Add "clonable" to the dictionary

This is used by ShadowRoot already.

whatwg.org/style-guide

@@ -42,6 +42,7 @@
  <li>canceled</li>
  <li>canceling</li>
  <li>cipher</li>
+ <li>clonable</li>
  <li>color space</li>
  <li>email</li>
  <li>filename</li>