refactor(core): add extension method to accept kotlin classes in ViewModelProviders.get

[ShirleyNekoDev]

Fixes: #22

Checklist

• [x] Commit messages follow conventionalcommits.org
• [ ] Tested on actual devices
• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

Changes

• see title

[JonasWanke]

65 adds the extension method viewModel() which simplifies this and even allows to specify a custom factory