[go_router] Fix replace routes hanging the originating completer

[nouvist]

The behavior of replace and pushReplacement involves popping the last route and ignoring the completer, leaving the last route's future unresolved. This PR introduces new behavior for both methods to inherit the last route's completer if the last route is an ImperativeRouteMatch.

Expected issue(s) should be fixed by this:

• Awaiting a GoRoute.push() is not resolving when the push page gets replaced via pushReplacement flutter#141251

Previous behavior:

sebelum.mp4

This PR behavior:

sesudah.mp4

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