feat: improved Link prefetching behavior, API, and docs; make it similar to Next.js

[jordanw66]

Overview

This PR makes prefetch on Link similar to prefetch on Next.js:

• Prefetching occurs by default using on:qvisible.
• Opt-out completely with prefetch={false}.
• Or, optionally, only prefetch javascript bundles (and not data) to render the page with prefetch="js".
• In dev mode, prefetch will occur with mouseover or focus instead of qvisible.
• If a user has their data saver setting set, prefetching is skipped.
• Optimized to not prefetch nor attach handlers on same-page Link. (hashes, navbar, etc)
• Added detailed jsdoc/API explanation of what prefetch does and how it works, along with options.
• Removed aria-pressed because this is for toggle buttons, not <a> tags. Screen readers already handle links, in fact adding and removing aria-pressed will cause confusion for these users as it's not normal link behavior.
• Improved the reliability of pathname comparison by normalizing path trailing slashes first, added unit tests for this.
• Added and modified unit tests for verifying decisions from prefetch checks.

Prefetching symbols (and/or data) to render a page before SPA navigation is a huge boost to user experience, especially on slow/mobile connections, preventing a burst of network activity during navigation.

If you think defaults or API should be different let me know.

References:
https://nextjs.org/docs/pages/api-reference/components/link#prefetch
https://nextjs.org/docs/messages/prefetch-true-deprecated
https://www.builder.io/blog/prefetching-nextjs-visual-guide
https://www.accessibility-developer-guide.com/examples/sensible-aria-usage/pressed/#intended-use

What is it?

• Feature / enhancement
• Bug
• Docs / tests / types / typos

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Use cases and why

• 1. One use case
• 2. Another use case

Checklist:

• My code follows the developer guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• Added new tests to cover the fix / functionality

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Name	Link
🔨 Latest commit	7376b03

[wmertens]

@jordanw66 this is great! Could I implore you to rebase?

An e2e test would be very nice too, just not quite sure how to test prefetching.

[wmertens]

@jordanw66 wow, I'm glad you're back now! Totally understand, if you could do a quick rebase that would be fine.

[wmertens]

Closing in favor of #5480