[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.

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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 "/"

[chunhtai]

Do you think it may be useful to provide a way to access a detail route of a certain go route from a complete different go route?

[LukasMirbt]

I agree that could be a nice additional feature but I consider it out of scope for this PR.

[chunhtai]

how does this still have a location?

[LukasMirbt]

Having access to the value without ./ (e.g. person/p1 compared to ./person/p1) is useful in some situations. However, it might be confusing for it to be called "location". I renamed it to "subpath" instead.

[LukasMirbt]

Update

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