Update parser to support <t>, <template>, /> syntax

sky/specs/markup.md

 Sky Markup: Syntax
 ==================
 
 A Sky file must consist of the following components:
 
  1. If the file is intended to be a top-level Sky application, the
     string "```#!mojo mojo:sky```" followed by a U+0020, U+000A or
     U+000D character.
 
     If the file is intended to be a module, then the string "SKY", a
@@ skipping 144 matching lines @@
  - The contents of the element aren't placed in the Element itself.
    They are instead placed into a DocumentFragment that you can obtain
    from the element's "content" attribute.
 
 <script>
  - Blocks until all previous imports have been loaded, then runs the
    script, with either 'module' or 'application' as the first
    argument, the exports of any imports that have "as" attributes at
    this time passed in as subsequent arguments, and with "this" set to
    null.
-   TODO(ianh): could be something other than null?
 
 <style>
  - Adds the contents to the document's styles.
 
 <content>
 <content select="...">
  - In a shadow tree, acts as an insertion point for distributed nodes.
    The select="" attribute gives the selector to use to pick the nodes
    to place in this insertion point; it defaults to everything.
 
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 The following attributes are available on all elements:
 
 id="" (any value)
 class="" (any value, space-separated)
 style="" (declaration part of a Sky style rule)
 lang="" (language code)
 dir="" (ltr or rtl only)
 
 contenteditable="" (subject to future developments)
 tabindex="" (subject to future developments)