feat(useFavicon): Add link tag if it doesn't exist

[babu-ch]

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Unable to verify with unit test
I deleted the link tag on the demo and ran it to confirm that the link tag was added.

⚠️ Slowing down new functions

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As the VueUse audience continues to grow, we have been inundated with an overwhelming number of feature requests and pull requests. As a result, maintaining the project has become increasingly challenging and has stretched our capacity to its limits. As such, in the near future, we may need to slow down our acceptance of new features and prioritize the stability and quality of existing functions. Please note that new features for VueUse may not be accepted at this time. If you have any new ideas, we suggest that you first incorporate them into your own codebase, iterate on them to suit your needs, and assess their generalizability. If you strongly believe that your ideas are beneficial to the community, you may submit a pull request along with your use cases, and we would be happy to review and discuss them. Thank you for your understanding.

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Description

fix #3439

Additional context

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🤖 Generated by Copilot at f8142c7

Fixed a bug and improved compatibility for useFavicon function. The function now creates and sets the appropriate link element for the favicon if it does not exist in the document.

🤖 Generated by Copilot at f8142c7

• Fix favicon not applying if no link element exists (link)