Add currency code validator

• Created ISO 4217 validator using all officially assigned currency codes obtained from this list.

• Create a test suite with some valid and some invalid currency codes.

• Update README file adding the new validator.

Checklist

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

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Thanks for your contrib! 🎉

Thanks for your contrib! 🎉