Add the <t> element and ignore whitespace outside it.

We now only preserve the whitespace inside a <t> element inside
the parser. This removes the known n^2 from reattaching whitespace
which should make parsing and appending nodes faster. I also
removed the dead WhitespaceMode code from the parser, and made
the dom-seralizer.sky auto indent the markup so the test output
would be readable.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/867963006

examples/city-list/city-list.sky

@@ -89,10 +89,10 @@ module.exports.LetterHeaderElement = class extends SkyElement {
     #population {
       color: #AAA;
     }
-
   </style>
   <div>
-    <span id="name">{{ name }}</span>,
+    <span id="name">{{ name }}</span>
+    <t>, </t>
     <span id="population">population {{ population }}</span>
   </div>
 </template>