[go_router]Show ShellRoute's routes in debug log - #3664

Fixes flutter/flutter#113819

Fix ShellRoute's sub routes to log output when debugLogDiagnostics is true.
In example/lib/shell_route.dart, the following log is output.

[GoRouter] Full paths for routes:
             => /a
             =>   /a/details
             => /b
             =>   /b/details
             => /c
             =>   /c/details

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