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/Columns/InsertColumns.cs

@@ -0,0 +1,40 @@
+using ClosedXML.Excel;
+using System;
+using System.Linq;
+
+namespace ClosedXML_Examples.Columns
+{
+    public class InsertColumns : IXLExample
+    {
+        public void Create(String filePath)
+        {
+            var workbook = new XLWorkbook();
+            var ws = workbook.Worksheets.Add("Inserting Columns");
+
+            // Color the entire spreadsheet using columns
+            ws.Columns().Style.Fill.BackgroundColor = XLColor.LightCyan;
+
+            // Put a value in a few cells
+            foreach (var r in Enumerable.Range(1, 5))
+                foreach (var c in Enumerable.Range(1, 5))
+                    ws.Cell(r, c).Value = "X";
+
+            var blueColumn = ws.Column(2);
+            var redColumn = ws.Column(5);
+
+            blueColumn.Style.Fill.BackgroundColor = XLColor.Blue;
+            blueColumn.InsertColumnsAfter(2);
+
+            redColumn.Style.Fill.BackgroundColor = XLColor.Red;
+            redColumn.InsertColumnsBefore(2);
+
+            ws.Rows(3, 4).Style.Fill.BackgroundColor = XLColor.Orange;
+            ws.Range("B1:D1").InsertColumnsAfter(2);
+            ws.Range("B2:D2").InsertColumnsBefore(2);
+            ws.Range("B3:D3").InsertColumnsAfter(2);
+            ws.Range("B4:D4").InsertColumnsBefore(2);
+
+            workbook.SaveAs(filePath);
+        }
+    }
+}