Initial commit of the ClosedXML library

Establishes the foundational structure, core Excel functionality, and tooling.

- Implements base components for cells, ranges, columns, and rows.
- Integrates a custom calculation engine with a wide range of Excel functions.
- Adds comprehensive support for styling, conditional formatting, data validation, comments, pictures, and charts.
- Configures build setup, project metadata (NuGet), and developer guidelines.
- Includes editor and Git attributes for consistent code style and line endings.

ClosedXML_Examples/Loading/ChangingBasicTable.cs

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+using System.IO;
+using ClosedXML.Excel;
+
+
+namespace ClosedXML_Examples
+{
+    public class ChangingBasicTable : IXLExample
+    {
+        public void Create(string filePath)
+        {
+            string tempFile = ExampleHelper.GetTempFilePath(filePath);
+            try
+            {
+                new BasicTable().Create(tempFile);
+                var workbook = new XLWorkbook(tempFile);
+                var ws = workbook.Worksheet(1);
+
+                // Change the background color of the headers
+                var rngHeaders = ws.Range("B3:F3");
+                rngHeaders.Style.Fill.BackgroundColor = XLColor.LightSalmon;
+
+                // Change the date formats
+                var rngDates = ws.Range("E4:E6");
+                rngDates.Style.DateFormat.Format = "MM/dd/yyyy";
+
+                // Change the income values to text
+                var rngNumbers = ws.Range("F4:F6");
+                foreach (var cell in rngNumbers.Cells())
+                {
+                    string formattedString = cell.GetFormattedString();
+                    cell.DataType = XLDataType.Text;
+                    cell.Value = formattedString + " Dollars";
+                }
+
+                ws.Columns().AdjustToContents();
+
+                workbook.SaveAs(filePath);
+            }
+            finally
+            {
+                if (File.Exists(tempFile))
+                {
+                    File.Delete(tempFile);
+                }
+            }
+        }
+    }
+}

ClosedXML_Examples/Loading/LoadFiles.cs

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+using System;
+using System.IO;
+using ClosedXML.Excel;
+
+namespace ClosedXML_Examples
+{
+    public class LoadFiles
+    {
+        public static void LoadAllFiles()
+        {
+            foreach (var file in Directory.GetFiles(Program.BaseCreatedDirectory))
+            {
+                var fileInfo = new FileInfo(file);
+                var fileName = fileInfo.Name;
+                LoadAndSaveFile(Path.Combine(Program.BaseCreatedDirectory, fileName), Path.Combine(Program.BaseModifiedDirectory, fileName));
+            }
+        }
+
+        private static void LoadAndSaveFile(String input, String output)
+        {
+            var wb = new XLWorkbook(input);
+            wb.SaveAs(output);
+            wb.SaveAs(output);
+        }
+    }
+}