Add page for Viewer

[ryantrem]

• Add a code block that works with gists.
• Add a viewer block that uses the new Viewer.
  • NOTE 1: The viewer is being pulled in using jsdelivr CDN. We could switch to our CDN if we publish the viewer dist somewhere.
  • NOTE 2: I'm pulling in a specific version rather than just @preview because the chunk names can change from version to version which means browser caching can break things.
• Add a "Viewer" menu entry right below "Babylon Native".
• Add a Viewer page that has a live demo (using the UFO model, which has multiple animations and audio, so demonstrates a lot of the current viewer features), a simple code snippet showing how simple it is to use the viewer in an html page, and some bullet points highlighting the qualities of the new viewer.
• Made some changes to launch.json, tasks.json, and package.json to simplify dev workflow (just hit F5 to run and test).