[go_router_builder] Use a mixin instead of an extension (#9275)
first step of fixing [#106790](https://github.com/flutter/flutter/issues/106790)
This PR changes the generation to use a mixin instead of an extension. The plan is to add the routing methods directly to GoRouteData. An extension won't be able to override the class implementations, so I changed it to a mixin.
I'm not sure about the versioning, so feel free to suggest changes.
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