Restore adaptive nature to new Scrollbar

[Piinks]

This PR makes the Scrollbar widget adaptive again.
A large refactoring was completed in #71664, which temporarily disabled the adaptive behavior of the Scrollbar widget. This brings that ability back, in a cleaner way by making the Scrollbar widget a wrapper class. The new Material Design scroll bar is in _MaterialScrollbar, keeping it separate from the CupertinoScrollbar.

The adaptability tests that were removed in #71664 have been restored. Added a todo to update the CupertinoScrollbar for desktop behavior.

Part of #70866

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[HansMuller]

LGTM

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• The status or check suite Windows tool_tests has failed. Please fix the issues identified (or deflake) before re-applying this label.

[AMH8790]

Mahmas501gmail.com