Add shallow routing

[webJose]

Sveltekit provides this feature named Shallow routing. In a nutshell, specifying an empty string as URL, the current URL is used to push or replace state. The concept itself is only useful with pushState, though.

Technical Details

To achieve this, Location.navigate() needs to account for the special case of the empty string, in which case the current URL is pushed/replaced with the provided state data.

It is like 95% done already.