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/Misc/CopyingWorksheets.cs

@@ -0,0 +1,44 @@
+using ClosedXML.Excel;
+using ClosedXML_Examples.Tables;
+using System.IO;
+
+namespace ClosedXML_Examples.Misc
+{
+    public class CopyingWorksheets : IXLExample
+    {
+        public void Create(string filePath)
+        {
+            string tempFile1 = ExampleHelper.GetTempFilePath(filePath);
+            string tempFile2 = ExampleHelper.GetTempFilePath(filePath);
+            try
+            {
+                new UsingTables().Create(tempFile1);
+                var wb = new XLWorkbook(tempFile1);
+
+                var wsSource = wb.Worksheet(1);
+                // Copy the worksheet to a new sheet in this workbook
+                wsSource.CopyTo("Copy");
+
+                // We're going to open another workbook to show that you can
+                // copy a sheet from one workbook to another:
+                new BasicTable().Create(tempFile2);
+                var wbSource = new XLWorkbook(tempFile2);
+                wbSource.Worksheet(1).CopyTo(wb, "Copy From Other");
+
+                // Save the workbook with the 2 copies
+                wb.SaveAs(filePath);
+            }
+            finally
+            {
+                if (File.Exists(tempFile1))
+                {
+                    File.Delete(tempFile1);
+                }
+                if (File.Exists(tempFile2))
+                {
+                    File.Delete(tempFile2);
+                }
+            }
+        }
+    }
+}