fix(ga4): be robust against missing navigator.sendBeacon

[voidus]

Closes #56.

For a more radical approach, see #58

[jahilldev]

Thanks for the contribution, @voidus 👍

As discussed in the linked issue, if you have time, could we add a fallback method using fetch with keepalive: true so that we're still recording metrics.

Something simple like this should do:

const queryParams = getQueryParams(trackingId, { type, event, debug });
const endpoint = window.minimalAnalytics?.analyticsEndpoint || analyticsEndpoint;

if(navigator.sendBeacon) {
  navigator.sendBeacon(`${endpoint}?${queryParams}`);
} else {
  fetch(`${endpoint}?${queryParams}`, {
    method: 'POST',
    keepalive: true
  });
}

bindEvents(trackingId);
trackCalled = true;