[Feature] Add support for images

[andrzejchm]

This PR adds support for reading and writing images in Excel (.xlsx) files. The implementation:

Key Features:

• Support for reading images from existing Excel files
• Support for adding new images to Excel files

Technical Implementation:

• Adds ImageCellValue for representing images in cells
• Creates dedicated _ImageCellParser for reading images from Excel files
• Creates dedicated _ImageCellCreator for writing images to Excel files
• Handles Excel's internal image storage format and relationships
• Properly manages Excel's drawing relationships and media files
• Supports EMU (English Metric Unit) conversion for accurate image sizing

Example usage:

// Reading an image from a cell                                                                                                                                                                                                                     
final imageCellValue = excel.sheets['Sheet1']!.cell(CellIndex.indexByString("A1")).value as ImageCellValue;                                                                                                                                          
final imageBytes = imageCellValue.bytes;                                                                                                                                                                                                            
final imageFormat = imageCellValue.format; // "png", "jpg", etc.                                                                                                                                                                                    
                                                                                                                                                                                                                                                    
// Writing an image to a cell                                                                                                                                                                                                                       
final imageBytes = File('image.png').readAsBytesSync();                                                                                                                                                                                             
final imageCellValue = ImageCellValue(bytes: imageBytes, format: 'png');                                                                                                                                                                            
excel.sheets['Sheet1'].cell(CellIndex.indexByString("A1")).value = imageCellValue;     

I've augmented excel_example.dart and excel_test.dart files with image support showcase/testing.