This PR adjusts the loader to include default config when enabling debug/performance/replay.

We should probably do a bit more testing etc. for this, as this is not really covered well as of now.
I did some tests locally with different scenarios, where stuff worked well, as far as I can tell.

Lazy mode

Lazy mode is only available for errors-only config. Users can manually opt-out of this by adding data-lazy="no" to their script tag.

In lazy mode, nothing happens until an error occurs. Once an error occurs, we download the CDN bundle, and then...:

• If onLoad is configured, trigger it
• If SDK.init() has not been called, call it with defaults

Eager mode

If either performance or replay is enabled, we are always in eager (non-lazy) mode. In this mode, we'll automatically load the CDN bundle, and then...:

• If onLoad is configured, trigger it
• If SDK.init() has not been called, call it with defaults

Defaults

The following default config is defined:

• dsn: Always set
• debug: true is set when debug bundles are enabled
• tracesSampleRate: 0.1 is set when performance is enabled
• replaysSessionSampleRate: 0.1 & replaysOnErrorSampleRate: 1 are set when replay is enabled
• integrations: Will add Sentry.BrowserTracing and/or Sentry.Replay if performance/replay is enabled

This config can be overwritten by the user, and is merged. So e.g. if they do:

Sentry.onLoad(function() {
  Sentry.init({
    tracesSampleRate: 0.5
  });
});

For a full loader build with every option enabled, the actual config will be:

{
  dsn: 'your-dsn',
  debug: true,
  tracesSampleRate: 0.5,
  replaysSessionSampleRate: 0.1,
  replaysOnErrorSampleRate: 1,
  integrations: [new Sentry.BrowserTracing(), new Sentry.Replay()]
}

Closes #46368