Add Magic detection for Base91 operations

- Add detection pattern for From Base91 operation in OperationConfig.json
- Pattern detects strings with 20+ characters using Base91 alphabet
- Enables automatic detection of Base91-encoded data by Magic operation
- Uses regex for Base91 character set detection
- Allows CyberChef AI to automatically suggest Base91 decoding

This enhancement makes Base91 operations discoverable through
CyberChef's Magic operation for automatic encoding detection.

Author: ialejandrozalles