fix(MU): account number is numeric (12!n) per ISO 13616

The SWIFT IBAN Registry BBAN structure for Mauritius is 4!a2!n2!n12!n3!n3!a,
so the 12-character account number is digits only. It was declared as
CharacterType.c (alphanumeric), which the file's own comment already records
as 12!n, so an IBAN with a letter in the account-number field validated as
true even though it is structurally invalid (and its check digits are correct).

Narrow the account-number class to CharacterType.n. The genuine registry
example still validates.

Author: spokodev

src/bbanStructure.ts

@@ -584,7 +584,7 @@ export class BbanStructure {
     [CountryCode.MU]: new BbanStructure(
       BbanStructurePart.bankCode(6, CharacterType.c), // 4!a2!n
       BbanStructurePart.branchCode(2, CharacterType.n),
-      BbanStructurePart.accountNumber(12, CharacterType.c),
+      BbanStructurePart.accountNumber(12, CharacterType.n),
       BbanStructurePart.accountType(3, CharacterType.n),
       BbanStructurePart.currencyType(3, CharacterType.a),
     ),

test/iban.spec.ts

@@ -11,6 +11,14 @@ describe("IBAN", () => {
     it("bad iban", () => {
       expect(IBAN.isValid("BA391990440001200278")).toBe(false);
     });
+    it("MU account number must be numeric (12!n)", () => {
+      // SWIFT IBAN Registry MU BBAN structure is 4!a2!n2!n12!n3!n3!a; the
+      // account number is 12 digits. A letter in that field is structurally
+      // invalid even when the check digits are correct (mod-97 == 1 here).
+      expect(IBAN.isValid("MU84BOMM010110103030020A000MUR")).toBe(false);
+      // Genuine registry example stays valid.
+      expect(IBAN.isValid("MU17BOMM0101101030300200000MUR")).toBe(true);
+    });
   });
 
   describe("Test check digit", () => {