Scribbly-Fun/Scribbly.Stencil

AI Summary: Implement a new attribute for Scribbly.Stencil that, when applied to an endpoint, generates constant string values representing the endpoint's route name and route template. These constants should be accessible to other parts of the application, enabling features like using them in `Results.CreateAt` calls to generate links to other endpoints.

AI Summary: Implement an optional builder extension method, `AddStencil`, for the Scribbly.Stencil library. This extension should allow users to register their endpoint groups and classes with the dependency injection (DI) container. The existing `new()` instantiation should be replaced with DI resolution when the extension is used, enabling dependency injection into endpoint and group classes.