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_Sandbox/ReflectionExtensions.cs

@@ -0,0 +1,17 @@
+#if _NET40_
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+using System.Text;
+
+namespace ClosedXML_Sandbox
+{
+    internal static class ReflectionExtensions
+    {
+        public static void SetValue(this PropertyInfo info, object obj, object value)
+        {
+            info.SetValue(obj, value, null);
+        }
+    }
+}
+#endif