fix: allow an opaque response in ping()

[jstarpl]

When using the library to connect to a Meshtastic radio, the Ping method attempts to fetch from hotspot-detect.html. Unlike the /api endpoints, the Meshtastic device HTTP server doesn't set any CORS headers on the hotspot-detect.html response. As a result, the PING will fail:

Since meshtasticjs doesn't actually check the response from the server in this instance - just needs the request to go through, we can use mode: no-cors and allow this fetch to resolve successfully.

[CLAassistant]

All committers have signed the CLA.

[sachaw]

Nice fix, strange thing is, the firmware should be setting those headers, so idk, either way this works great.