fix(hmr): infer hmr hostname on client

[stevenpwaters]

Github/VS Codespaces don't forward WebSocket requests so the HMR hostname must be manually configured in vite config like so:

hmr: {
  port: 443,
  hostname: '{uuid}-{port}.apps.codespaces.githubusercontent.com'
}

New codespaces get a different UUID so need to be reconfigured each time. This PR infers hostname on the client rather than server by default so codespace users only need to specify the WS port (443) in vite config.

This is the same behaviour as prior to bec6b3c. Use cases such as #918 can now be addressed by defining a hostname in the config as above (introduced subsequently with b753478); these users are less inconvenienced as their backend URL is unlikely to change.