Prepare flutter_duit_cupertiono package layout

### Issue description

The Flutter team [plans](https://github.com/flutter/flutter/issues/101479) to separate the material and cupertino widget libraries and publish them as independent packages.

To keep the Duit codebase up to date and take advantage of the library separation in the future, all material widgets need to be moved into a separate package.

### When working on an issue, it is important to consider:
- Package architecture considering experimental support for external UI libraries
- Think through and lay out the library structure (how, where, and what files will be stored)
- Preparing the package for publication: linter, tests, readme, contribution guide, changelog, etc.