Add support for Mauritius TIN

[unho]

• https://www.oecd.org/tax/automatic-exchange/crs-implementation-and-assistance/tax-identification-numbers/Mauritius-TIN.pdf

  In Mauritius the TIN is officially called the Tax Account Number (TAN).
  For individuals: The TAN consists of 8 numerals (format: 99999999). The first digit of the TAN is always 1, 5, 7 or 8.
  For entities: The TAN consists of 8 numerals (format 99999999). The first digit of the TAN is always 2 or 3.

• https://world.salestaxhandbook.com/mu-mauritius

  format is VAT99999999.

• http://servicesmns.mu/forms/CNP2019/CNP_YrlyTDS2019_CSV_FileLayout_v1.0.pdf

  Tax Account Number (TAN): It consists of numbers 0 to 9 only and its length is 8.

• https://www.mcb.mu/online-banking/ib-demo/How-to-perform-MRA-Payment-through-IB/_files/1024x768/Data/p92273/XMLFiles//steps1.xml

  (Tax Account Number) comprises of eight (8) digits only.

• https://www.esunbank.com.tw/bank/-/media/esunbank/files/about/announcement/overseas/hong-kong/crs_tin.pdf

  Individual: 8 numerals (format: 99999999). The first digit of the TAN is always 1, 7, 8.
  Entity: 8 numerals (format 99999999). The first digit of the TAN is always 2 or 3.

• According to https://eservices3.mra.mu/ctaxreturn/ctaxreturn.jsp it is 8 digits and it is called Tax Account Number (TAN).
• https://tin-check.com/en/tin-check-mauritius/

  The TIN is called Tax Account Number (TAN).
  For individuals: The TIN consists of 8 digits (the 1st digit is always 1, 7 or 8).
  For entities: The TIN consists of 8 digits (the 1st digit is always 2 or 3).

Examples (I could only find 2 examples):

• 20405553 from https://c-contracting.mu/about-us/
• 18421000 from http://www.uniconsults.mu/uniconsults/images/SPay/Spay%20Statement_of_emoluments.pdf