Sync isbn-verifier with problem specifications

[ManasDasri]

Adds two canonical test cases missing from the isbn-verifier suite: 'only one check digit is allowed' and 'X is not substituted by the value 10'. Verified via ./bin/test.ps1 isbn-verifier - 21/21 tests pass. No changes to Example.vb needed since the existing solution already handles both cases correctly.

Closes #453

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[BNAndras]

Looks good! Thanks!