Allow running behind a reverse proxy in a subpath

[dzil123]

this is a collection of tweaks I made to improve the experience of using wasm-server-runner behind vscode’s port forwarding, where wasm-server-runner is exposed at https://host/proxy/1334/ instead of http://host:1334/

• use relative urls everywhere, to allow running in a subpath
  • new browsers support https and relative urls in the websocket constructor, making the wss:// logic unnecessary
• remove extra newline from forwarded console log messages
• check fetch response http status in startReloadInterval
  • fixes behavior when the reverse proxy returns http 500 status