[go_router] Fixed TabView swiping in custom stateful shell route example

[tolo]

Updated custom_stateful_shell_route.dart example to better support swiping in TabView. Also added code to demonstrate use of PageView instead of TabView. Note that to be fully effective from a usability perspective, the PR #6467 (branch preloading) need also be merged.

This PR addresses:

• [go_router] In TabBarView with StatefulShellBranch, content remains Blank when swiping to switch between tabs flutter#150837
• [go_router] Add support for the TabView for more beautiful animation flutter#112267

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[chunhtai]

Code LGTM, Can you add a test to make sure the fix is not reverted in the future?

[tolo]

I see some confusion in issues around go_router related to using CupertinoTabScaffold, so I thought I'd include that as well in the sample, since it's somewhat related. Perhaps it can contributed to remove some confusion. Let me know what you think.

[tolo]

Pushed an additional update with some cleanup & clarifications

[chunhtai]

LGTM

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