Enhancement: add TypeScript generics support for custom renderer return types

[chu0119]

Feature request

The marked library currently uses loose TypeScript types for custom renderers. Adding proper generics for return types would improve type safety when extending the renderer.

Proposed solution

Allow renderer methods to specify their return type via generics, enabling type-safe custom rendering.

[UziTech]

That would be great! There have been a couple attempts at this but no one has finished a working PR for it. If you would like to try, that would be great 😁👍