Allow navigation from to be executed from the app router

[Alxandr]

Currently, if you do router.navigate('foo'), it redirects you relative to that router (as it logical), but there should be some way to tell it to redirect you to foo relative to root. I suggest this can be done in two manners. One, with the options object passed to navigate (something like router.navigate('foo', {useAppRouter: true})), and router.navigate('/foo') where the slash at the beginning indicates I want it to be from the root.

I also think that the useAppRouter should take precedence over the slash start though, so that if you explicitly set useAppRouter: false, and pass in /foo it would still redirect relative to the router. I can't really picture a use-case for it, but I'm sure somebody can, and I don't really see a reason it shouldn't be supported.

[Alxandr]

As far as I can see, your commit does not deal with navigate('/foo') (url's starting with /).

[EisenbergEffect]

No, I'm not changing the '/' aspect at this time. I will consider this for the future.

[Alxandr]

Fair enough. I'll make a new issue for it then, so it at least get's tracked.