[fix] Invalidating a route now fires sveltekit:navigation-start event

[henninggross]

Closes #2528

By now also calling router.notify() when invalidating an URL it is made sure that the sveltekit:navigation-start event is fired upon invalidation.
This way one can for example show a loading indicator when trying to reload the same page.

I am calling router.notify() instead of just dispatching the sveltekit:navigation-start event to also update navigating.

Other than that it is just more consistent to have a start event when also having an end event as one could expect both to be fired the same way they are when navigating normally.

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[benmccann]

This doesn't seem quite right to me because, if the page URL doesn't change, I'm not sure we should be firing navigation-start and navigation-end

[benmccann]

Thanks for this. I took another stab at it in #2649