feat: Introduce StatefulShellRoute

[HironobuIga]

Description

• Introduce StatefulShellRoute

Fixes #91

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• [x] checking tab and drawer transitions in Android
• [x] checking tab and drawer transitions in iOS
• [x] checking drawer transitions in Web

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] My code has been formatted with flutter format lib and flutter pub run import_sorter:main
• [x] My code has been fixed with dart fix --apply lib
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules

[HironobuIga]

Thanks for the PR! I commented.

Thank you for your review. I handled your comments.

[HironobuIga]

@koji-1009 I handled your comments. Could you check again?

[github-actions[bot]]

Commented by GitHub Bot

PR: #142 preview link