feat: autocapture unhandled exceptions

[ioannisj]

💡 Motivation and Context

Part of PostHog/posthog#38845

Docs: PostHog/posthog.com#13539

Adds autocapture for Flutter framework and Dart platform unhandled exceptions

💚 How did you test it?

• Unit tests: TODO

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[ioannisj]

Still need to address some feedback on #212, so just looking for a general review on the autocapture approach/config for now 🙏

[marandaneto]

I'd add tests for the new handlers eg isolate etc
otherwise LGTM

[ioannisj]

I'd add tests for the new handlers eg isolate etc otherwise LGTM

Tried adding some but could not find a good way of unit testing these. I'll give it another shot

[github-actions]

	Fails
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## Next
- autocapture unhandled exceptions ([#214](https://github.com/PostHog/posthog-flutter/pull/214))

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