Flutter macOS support

[marandaneto]

:bulb: Motivation and Context

Relates to https://github.com/PostHog/posthog-flutter/issues/51

:green_heart: How did you test it?

Running the example on macOS

:pencil: Checklist

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

[marandaneto]

Waiting https://cocoapods.org/pods/PostHog shows the RC.1 version before merging