feat: Add sheet context to importSheets callback

[StarkOne]

Add sheet name to importSheets callback function

Description

This PR enhances the importSheets method by allowing callback functions to receive the current sheet name as the first parameter. This is useful for data validation, especially when the same field names exist across different sheets.

The implementation adds a new withSheetContext() method that, when called, will make importSheets pass the sheet name as the first parameter to the callback function.

@rap2hpoutre This PR solves that issue #366

Changes

• Added $with_sheet_context property to the Importable trait
• Added withSheetContext() method to enable the feature
• Modified importSheet() method to conditionally pass sheet name to callback
• Updated facade documentation with the new method

Backwards Compatibility

This change maintains full backwards compatibility:

• Existing code continues to work without modification
• New functionality is opt-in via the withSheetContext() method

Usage Example

$collections = (new FastExcel)
    ->withSheetContext()
    ->importSheets('file.xlsx', function ($sheetName, $row) {
        // Perform sheet-specific validation
        if ($sheetName === 'Users' && empty($row['email'])) {
            return null; // Skip rows without email in Users sheet
        }
        
        // Add sheet information to result
        $row['_sheet'] = $sheetName;
        
        return $row;
    });