[go_router_builder]: Add support for relative routes

[LukasMirbt]

Adds support for TypedRelativeGoRoute.

Builds on #9732

Continuation of #8476 by @ThangVuNguyenViet

Compared to #8476, the approach taken in this PR avoids breaking changes and creates a separate RouteData subclass instead of extending GoRouteData. The approach increases flexibility and avoids extending behavior that might not make sense for a relative route.

Fully resolves flutter/flutter#108177.

Pre-Review 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.
• 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 I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[gemini-code-assist]

Code Review

This pull request introduces support for relative routes with TypedRelativeGoRoute. I've added one suggestion to improve validation by ensuring that paths for relative routes are not absolute, which will help catch errors early during development.

[LukasMirbt]

Update

• Factored out repeated logic between GoRouteConfig and RelativeGoRouteConfig with a mixin
• Added readme section
• Added build-time validation preventing the entire sub-tree of a TypedRelativeGoRoute from containing a TypedGoRoute (all other types are valid)
• Added build-time validation preventing a TypedRelativeGoRoute path from starting with "/"

[LukasMirbt]

Update

• Added to readme explaining the purpose of relative routes
• Bumped minor version
• Renamed confusingly named RelativeGoRouteData location getter to subpath

[chunhtai]

LGTM