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/MultipleSheets.cs

@@ -0,0 +1,30 @@
+using System.Linq;
+using ClosedXML.Excel;
+
+
+namespace ClosedXML_Examples.Misc
+{
+    public class MultipleSheets : IXLExample
+    {
+        
+        public void Create(string filePath)
+        {
+            var wb = new XLWorkbook();
+            foreach (var wsNum in Enumerable.Range(1, 5))
+            {
+                wb.Worksheets.Add("Original Pos. is " + wsNum.ToString());
+            }
+
+            // Move first worksheet to the last position
+            wb.Worksheet(1).Position = wb.Worksheets.Count() + 1;
+
+            // Delete worksheet on position 4 (in this case it's where original position = 5)
+            wb.Worksheet(4).Delete();
+
+            // Swap sheets in positions 1 and 2
+            wb.Worksheet(2).Position = 1;
+
+            wb.SaveAs(filePath);
+        }
+    }
+}