VS Code Extension: Add support for retrieving token from environment variable

When running in a DevContainer or Codespace, we can set secrets to be passed into the environment when starting the container. The extension should support grabbing the auth token from a variable like CIRCLECI_TOKEN or similar. Especially since it sometimes seems to lose the token after the codespace or container is restarted.

To make this more complete, an additional variable like CIRCLECI_SERVER to get the server URL for enterprise customers would be needed as well.