Implement a test server

[fwcd]

Make the lighthouse server URL customizable and add a simple test server with a web interface that can replace the official lighthouse server for testing purposes (LighthouseTestServer).

To do:

• Add test server target
• Implement test server
  • Add web interface (which includes a basic client that renders displays to a canvas)
  • Handle /websocket endpoint on the server side
• Use async/await in the test server (specifically in the ConnectionHandler)
  • Fix macOS build (there seem to be a few differences in how Linux and macOS Vapor handle asynchrony, we should address these)
• Document how to run test server (swift run LighthouseTestServer) and how to use it (--url ws://localhost:8080/websocket arg for demo etc.)
• Perhaps modularize into multiple packages so consumers that just need the lighthouse client don't have to pull the Vapor dependencies for the test server?