Add example breadcrumbs

[jamesblasco]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Example of how Breadcrumbs could look like. There are still some changes needed as matches variable is marked as visibleForTesting

Simulator.Screen.Recording.-.iPhone.13.-.2021-12-22.at.20.07.50.mp4

It also showcase how it would look the Cupertino Back Button with the ContextMenu that displays previous routes

Simulator.Screen.Recording.-.iPhone.13.-.2021-12-22.at.20.14.46.mp4

⤵️ What is the current behavior?

🆕 What is the new behavior (if this is a feature change)?

💥 Does this PR introduce a breaking change?

🐛 Recommendations for testing

📝 Links to relevant issues/docs

🤔 Checklist before submitting

• I made sure all projects build.
• I updated CHANGELOG.md to add a description of the change.
• I updated the relevant documentation.
• I updated the relevant code example.
• I rebased onto current master.

[csells]

This PR isn't done until all of the ignore lines from the examples are dropped.

[csells]

Also, I really love the breadcrumbs example but I don't think the cupertino_back_button example shows off anything new and it's not as obvious when you run it what's going on. I think we keep the breadcrumbs example but drop that cupertino_back_button example. Thoughts?

[csells]

I finally, I think there needs to be some docs talking about the new publicly available GoRouterMatch to left folks know how they can get to the stack of matches for their own uses.

Looking good!

[chunhtai]

Since this package has been moved to https://github.com/flutter/packages/tree/main/packages/go_router

Can you create a pull request against the flutter/packages instead?