`attachThreads` should attach native threads even if the exception comes from Dart/Flutter

If I enable the `attachThreads` option, I would expect that native threads are attached to my crash-reports, even if the crash reports come out of the Dart/Flutter layer.

One tricky issue, I can imagine, is that the timing of the reading of the active threads plays a significant role. Ideally, the native threads are read at the time when `Sentry.capture*` is called. Otherwise, it might not become as useful.

Other problems include: How do I see what's a native thread, and what's a Dart/Flutter thread (isolate).

```[tasklist]
### Blocked by
- [ ] https://github.com/getsentry/sentry-cocoa/issues/1870
```