add prerequisities for phone number exercise

[AnishDe12020]

I have added the prerequisites for the phone number exercise

references #984

[github-actions]

Dear AnishDe12020

Thank you for contributing to the JavaScript track on Exercism! 💙
You will see some automated feedback below 🤖. It would be great if you can make sure your PR covers those points. This will save your reviewer some time and your change can be merged quicker.

• ✍️ If your PR is not related to an existing issue (and is not self-explaining like a typo fix), please make sure the description explains why the change you made is necessary.

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• 📏 Make sure you set the appropriate x:size label for the PR. (This also works after merging, in case you forgot about it.)

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