fix(isCreditCard): anchor Mastercard pattern to 16 digits

[hong4rc]

isCreditCard('5108') returns true, and so does a 25-digit 5100000000000000000000003 — both wrongly accepted as Mastercard.

In the Mastercard pattern, | has the lowest precedence, so the 5x branch ^5[1-5][0-9]{2} is anchored only at the start with no length and no $ — any string that merely starts with a 51–55 prefix matches, regardless of length (Luhn doesn't bound length). Grouping the prefixes so the shared [0-9]{12}$ applies to both branches fixes it:

- mastercard: /^5[1-5][0-9]{2}|(222[1-9]|22[3-9][0-9]|2[3-6][0-9]{2}|27[01][0-9]|2720)[0-9]{12}$/,
+ mastercard: /^(5[1-5][0-9]{2}|(222[1-9]|22[3-9][0-9]|2[3-6][0-9]{2}|27[01][0-9]|2720))[0-9]{12}$/,

Tests: 5105105105105100 stays valid; 5108 and the 25-digit case are now rejected. npm test green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a38f15b) to head (c34b341).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2794   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2587      2587           
  Branches       656       656           
=========================================
  Hits          2587      2587           

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