[go _route] fragment parameter added

[AffanShaikhsurab]

Description of Change

This PR addresses the need for fragments/hashes to be treated as first-party parameters within the go_router package, as highlighted in issue #150155. Previously, users had to manually append the fragment to the URL, which could lead to potential bugs. With this update, the fragment is now a dedicated parameter, allowing for a more seamless and bug-free integration.

Before:

final location = context.namedLocation('some_route'); // const nested records
// Manually adding the fragment, hoping there aren't any weird bugs surrounding it
context.replace('$location#https://a.url/that?i=mightuse');

After:

// Directly passing the fragment as a parameter
context.goNamed("details", fragment: 'https://a.url/that?i=mightuse');

// or

final location = GoRouterState.of(context).namedLocation(
  'details',
  fragment: 'section3',
);
context.go(location);

// or 

final location = context.namedLocation(
 'details',
 fragment: 'section3',
);
context.go(location);

Issues Fixed

This PR resolves issue #150155.

---

Pre-launch Checklist

• [✔️ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [✔️ ] I read the Tree Hygiene page, which explains my responsibilities.
• [✔️ ] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• 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 linked to 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, or this PR is exempt from CHANGELOG changes.
• [✔️ ] 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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[AffanShaikhsurab]

@chunhtai @hannah-hyj Can you please tell me what improvements I should make?