[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

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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 RelativeGoRouteData location getter to subpath

[LukasMirbt]

@chunhtai Checks failing because the PR depends on go_router: ^16.2.0. Not sure what to do, please advise.

[LukasMirbt]

@chunhtai CI passing!