feat: expose event for navigation handler when navigation is popstate or link

[seo-rii]

Export event object of PopStateEvent into navigation handler callback if navigation is popstate.

Closes #13169.

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Preview: https://svelte-dev-git-preview-kit-14307-svelte.vercel.app/

[eltigerchino]

I'm wondering if it's worth exposing the entire event object instead of an individual property here. This would help us avoid this issue again if a new property is added.

[Rich-Harris]

yeah we definitely shouldn't just add hasUAVisualTransition. navigation.event seems reasonable (and we can presumably even narrow the event type on navigation.type), though is it weird that the type of Navigation['event'] isn't NavigationEvent?

[seo-rii]

@eltigerchino It seems more reasonable, so I've updated PR to expose entire event object.

[Rich-Harris]

Thank you. I think this is closer but still not quite ready to merge:

• The generic approach doesn't appear to give us any type safety benefits — if I refer to navigation.event inside a block like if (navigation.type === 'popstate') it should be typed as PopStateEvent. We need to use discriminated unions
• We should add event: PointerEvent to navigations with type: 'link'

[seo-rii]

• Sorry. I misunderstood that typescript could infer generic from fields.
• I kept delta/event to other navigation types as undefined to avoid breaking changes.
• I've added MouseEvent since link click event is click.