Added Anguilla mobile number validation (en-AI)

References:

• International Telecommunication Union

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)

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@rubiin -- once you have reviewed any PR and approved, you can add a ready-to-land tag that I can quick go through and put my approval and merge. Thanks for your reviews!