[go_router] add current state getter

[cedvdb]

Adds a current state getter that returns GoRouterState.

I've no idea if this is correct but the added tests seem to be passing. Rebuilding the state may be expensive too ?

Fix: flutter/flutter#129833

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• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[chunhtai]

This can also solve flutter/flutter#129833

Edit: nvm you already linked the issue