New Functions & .NET 8 Demo & Fix Excel Close Issue

What's Changed

• New functions & Fix Close & .NET 8 Demo by @fatihyildizhan in #21

// call save function
ExcelLabs.SaveFile(title, path, sheetName, cells);

// call safe save function
ExcelLabs.SaveFileWithCleanXmlText(title, path, sheetName, cells);

// clean not-allowed XML characters
string safeToWriteText = ExcelLabs.CleanTextForXml(title + " safe");
Console.WriteLine("Safe text: " + safeToWriteText);

// convert integer to Excel Column Letter like 1 to A
string excelColumnLetter1 = ExcelLabs.ColumnIndexToColumnLetter(1);
Console.WriteLine("1 to column letter: " + excelColumnLetter1); // A

// convert integer to Excel Column Letter like 1 to G
string excelColumnLetter7 = ExcelLabs.ColumnIndexToColumnLetter(7);
Console.WriteLine("7 to column letter: " + excelColumnLetter7); // G

Full Changelog: v2.2.1...v3.0.3