[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 resolvesflutter/flutter#108177.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• 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.
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[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 RelativeGoRouteDatalocation getter to subpath

[LukasMirbt]

@chunhtai CI passing!