Add viewer app export

[slimbuck]

This PR:

• adds an export option for generating a simple web viewer using https://github.com/playcanvas/web-components
• the app comprises 3 files (html, camera controller, splat) which are zipped using jszip
• the splat is exported in compressed ply format
• the viewer's background color is taken from the editor's current background color

Notes:

• We will likely add lots of export options in future for things like:
  • compressed or uncompressed ply
  • camera types: 1st person, orbit camera, camera path
  • support for AR mode
• I couldn't get JSZip to import correctly into typescript so it's loaded as an external script for now