Add environment variable with server URL

[ribrdb]

It would be really cool if code-server could put an env var into the terminals it starts that tells what url is being used to access the server.

In particular I want to use this in scripts that start servers, so I can print out http://localhost:8080 when running under normal vscode, but https://myserver.com/proxy/8080 when running under code server.

I'd also use this to configure the upstreamProxy setting of karma when running under code-server.

[code-asher]

I think this is a duplicate of #1510 although with a different use case. We'll add an environment variable with a name like CODE_SERVER_HTTP_REFERER.

I'll close so we can track on the other issue but thanks for opening this since it gives additional perspective on the original issue!