[go_router] Fixes crashes when popping navigators manually

[chunhtai]

fixes flutter/flutter#116784
fixes flutter/flutter#116769
fixes flutter/flutter#117431

The issue is that the app bar will pop the root navigator and skipping the navigator in the shellroute, and the goRouter only expect the RouteMatchList to be popped start from the last RouteMatch. This pr fixes it so that it can handle the case where developer may manually pop any navigator.

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[chunhtai]

We missed this when we refactor the pop. PopNavigatorRouterDelegateMixin is no longer needed since we override poproute directly

[leehack]

Can we also add a test case that pop multiple times?
I think this PR would fix flutter/flutter#117431 but just want to make sure of it.

I added a test case for it in this PR: #2989 but since it's close, I'd like to add it in this PR at least.

[chunhtai]

added on pop twice

[leehack]

Thanks ;)
I have no right to approve the PR but everything looks good to me.

[leehack]

#2989 could be added to the issue list since it's a potential fix for it as well.

[chunhtai]

looks like this is blocked on #2977

[devberkay]

This issue is also fixed with this pr.