reset navigation store to null after aborted nav

[Rich-Harris]

Fixes #4660. Instead of tracking which is the 'current' navigation, we track whether the current update is aborted. If not, we reset navigation to null after it completes.

In order for this to work when the user clicks on a link to /current-page during a navigation to /different-page, we need to stop treating /current-page clicks as no-ops. This was something we'd talked about doing anyway. (Next step is to provide more control over when load reruns.)

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