Migrate to PopScope and Fix back button not showing destination issue

[maheshj01]

Fix : #48 #49 #51

• Migrate WillPopScope to PopScope Issue 48
• Fix Deprecation Issue 49
• Migrate example app to Flutter's Gradle plugins
• Fix: Navbar Destination not updating on backbutton press Issue 51
• docs: Navbar Router should be wrapped in builder for Snackbar to work

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing using flutter test

If you need help, consider pinging the maintainer @maheshmnj