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_Tests/Examples/ColumnsTests.cs

@@ -0,0 +1,40 @@
+using ClosedXML_Examples;
+using ClosedXML_Examples.Columns;
+using NUnit.Framework;
+
+namespace ClosedXML_Tests.Examples
+{
+    [TestFixture]
+    public class ColumnsTests
+    {
+        [Test]
+        public void ColumnCells()
+        {
+            TestHelper.RunTestExample<ColumnCells>(@"Columns\ColumnCells.xlsx");
+        }
+
+        [Test]
+        public void ColumnCollections()
+        {
+            TestHelper.RunTestExample<ColumnCollection>(@"Columns\ColumnCollection.xlsx");
+        }
+
+        [Test]
+        public void ColumnSettings()
+        {
+            TestHelper.RunTestExample<ColumnSettings>(@"Columns\ColumnSettings.xlsx");
+        }
+
+        [Test]
+        public void DeletingColumns()
+        {
+            TestHelper.RunTestExample<DeletingColumns>(@"Columns\DeletingColumns.xlsx");
+        }
+
+        //[Test] // Not working yet
+        public void InsertColumns()
+        {
+            TestHelper.RunTestExample<InsertColumns>(@"Columns\InsertColumns.xlsx");
+        }
+    }
+}