feat: Ensure Bugsnag/user preload scripts can coexist
feat: Ensure Bugsnag/user preload scripts can coexist

The setPreloads() method completely replaces the existing scripts,
so check for existing before setting.

This also patchs setPreloads() after we have used it, to ensure
Bugsnag's preload script is always included first.

bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js