feat: infer type of the URLPattern context passed to the handler

[Danielduel]

Hi, I think the change is very visible when you open /examples/typesafe_parameters.ts.

I have few things missing in the typesafety of the internals - like few nevers that shouldn't be nevers and cleaning up few snippets that I was using to beat the typesystem to quickly check if everything is bueno.

I open this to get initial feedback if it is welcome or I should go and create something like safe fork packaged under different name (which is less preferred option for me).

I might add this to URLPattern itself in future, I will migrate rutt if my changes will get there, I felt like changing rutt will be far easier because the narrow scope of the lib.

Thanks!