breaking: let beforeunload handle links that will unload the document

[eltigerchino]

fixes #8597

The issue of the missing confirmation dialog stems from this code block that prevents the click event behaviour if we call cancel() in beforeNavigate. As a result, the navigation is never attempted and the beforeunload event never occurs. It needs to occur so that we can can call event.preventDefault() which displays the confirmation dialog.

kit/packages/kit/src/runtime/client/client.js

Lines 1550 to 1560 in 98e4b8f

	if (external || options.reload) {
	if (before_navigate({ url, type: 'link' })) {
	// set `navigating` to `true` to prevent `beforeNavigate` callbacks
	// being called when the page unloads
	navigating = true;
	} else {
	event.preventDefault();
	}
	return;
	}

The solution is to simply return in our click handler and delegate the responsibility to the beforeunload handler.

This PR also modifies an existing test to check for the beforeunload confirmation dialog.
However, beforeunload still doesn't occur when clicking on links in Safari #9747 (comment) so the macos tests are failing so the test is skipped for webkit

EDIT: upon re-reading #9747 the concerns mentioned in #9747 (comment) might mean this fix is not the one

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[Rich-Harris]

going to close this per #8597 (comment) — we want to be able to intercept external link clicks