Adds BrowserState to support parameter in web platform.

[chunhtai]

Previously the extra parameter is not stored in browser session. When the browser backward and forward, it only sends url to the GoRouter without extra parameter. It may crash if the the page expecting a extra parameter to build. This pr fixes it by introducing a new class BrowserState. If GoRouter sees this BrwoserState in the extra parameter, it will send it over to the browser so that the backward and forward can sends the same BrowserState back for the page.

fixes https://github.com/flutter/flutter/issues/109404

Pre-launch Checklist

• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• [ ] I signed the CLA.
• [ ] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [ ] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [ ] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [ ] 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.

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