fix: yield main thread before navigating

[Rich-Harris]

By default, SvelteKit preloads the data and code needed to navigate when you touch or hover over a link. As a consequence, it's often the case that navigation can occur immediately following a click event, with no delay for network activity.

Ironically, this means that navigation becomes less responsive, not more, because the browser doesn't get a chance to repaint before the navigation occurs and the page is re-rendered, which can cause dropped frames on slower devices. Because of this, the Interaction to Next Paint (INP) metric suffers, which harms the Core Web Vitals (CWV) score of SvelteKit apps.

This PR adds a tiny pause inside the click handler before navigation. The combination of requestAnimationFrame and setTimeout allows the browser to repaint, but without creating a user-noticeable delay.

No tests because I'm not sure how you would test for this.

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[benmccann]

I think this may cause errors to be ignored because addEventListener isn't async and doesn't accept a promise and so there's no rejection handler that gets applied in the error case