Add automatic credit card masking

[alkinbg]

Closes #2

What changed

Added automatic credit card number detection to Protect::protect().

Card numbers no longer need to be registered explicitly as sensitive values.
Candidates are detected in scalar values, validated using the Luhn checksum
and then passed through the existing Redact::redact() mechanism.

The implementation supports:

• 13–19 digit PANs
• numbers with spaces or hyphens
• multiple card numbers in the same value
• card numbers surrounded by other numeric data

I kept the detection separate from the actual redaction logic so existing
custom Redact callbacks continue to work unchanged.

Why the detection is a little more than a regex

A simple regex works for basic cases, but it becomes unreliable when other
numbers are next to the PAN.

For example:

4111 1111 1111 1111 123

or:

Order 123 4111 1111 1111 1111

A greedy match can include the extra digits, fail Luhn validation and leave
the actual card number unprotected.

_redactCreditCardSequence() therefore examines the numeric groups inside the
matched sequence and redacts only the valid PAN range.

False positives

The detector does not redact a value based only on its length.

Candidates must pass Luhn validation, and repeated-digit values such as
0000000000000000 are ignored.

Contiguous values longer than 19 digits are also left untouched, even when
they contain a valid PAN as a prefix.

Backward compatibility

One issue I noticed while implementing this was that running every scalar
through preg_replace_callback() converts non-string values to strings.

The implementation returns the original scalar when no redaction occurred, so
integers, floats and booleans keep their original type.

Tests

Added coverage for:

• Visa, Mastercard and Amex test numbers
• 13- and 19-digit boundaries
• space- and hyphen-formatted PANs
• PANs next to expiry/CVV/other numeric values
• multiple and adjacent PANs
• invalid Luhn values and other false positives
• preservation of non-string scalar types
• nested arrays
• custom Redact callbacks

Full test suite:

128 tests, 179 assertions