feat: Sliver toolbar

[GroovinChip]

This PR introduces the SliverToolbar widget, which allows developers to achieve effects like those desired in #321.

Closes #321

Pre-launch Checklist

• [x] I have incremented the package version as appropriate and updated CHANGELOG.md with my changes
• [x] I have added/updated relevant documentation
• [x] I have run "optimize/organize imports" on all changed files
• [x] I have addressed all analyzer warnings as best I could

[github-actions[bot]]

Dart Code Metrics analyze report of macos_ui. ✅

Summary

• Scanned folders: lib

• Total scanned files: 75

• Total lines of source code: 9105

• Total classes: 189

• Average Cyclomatic Number per line of code: 0.24 / 5

• Average Source Lines of Code per method: 11

• Total tech debt: not found

• Found issues: 0 ✅

[whiplashoo]

On a trip, will have a look soon!

[whiplashoo]

Superb! LGTM