Split the package for native Dart support

[TesteurManiak]

This is something I've been thinking about after seeing this issue on the repo we've initially forked: poitch/dart-matomo#41

The idea would be to move Flutter code and dependencies to a separate package (e.g.: matomo_tracker_flutter) and only keep pure Dart code in matomo_tracker. The biggest change would be to convert this repository into a monorepo (probably managed using melos) and redirect the userbase to the new Flutter package to keep using it easily inside their Flutter project.

This would allow for a better separation of responsibilities and make the whole logic useable inside 3rd party Dart frameworks.

[EPNW-Eric]

I agree that a separation would be nice, but I think it would take some effort to find a good solution.

A concern that comes to my mind is that there is a dependence on shared_preferences which is a flutter package. The dart-only-version would either have to provide a solution that works for flutter and non-flutter, or find some kind of abstraction (which optimally would then be implemented by the flutter version automatically, probably on the base of shared_preferences).

[TesteurManiak]

We could migrate from shared_preferences to hive (or isar when web support will work) as it is independent from the Flutter SDK.

As we already have the LocalStorage abstraction it wouldn't be too hard. (But not a priority for v4.0.0 maybe for a future v5.0.0)