[6.x] Configurable of Laravel dusk routes enhancement

[dmitryuk]

Continue declined PR #772
Resolve an issue #773

The issue I mean is Laravel dusk can't work with a separated backend and frontend.
For example, frontend (SPA) is located on "example.com", backend - "api.example.com".
Since Dusk's API internal methods will call example.com/_dusk/... methods, it is not reachable on this service.
So I searched a way how to configure it, but currently it is not possible. Routes are hardcoded.

I improved the PR by adding config file, so in future it can be used in future for resolving issues like #393