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/Program.cs

@@ -0,0 +1,29 @@
+using System;
+
+namespace ClosedXML_Sandbox
+{
+    internal static class Program
+    {
+        private static void Main(string[] args)
+        {
+            Console.WriteLine("Running {0}", nameof(PerformanceRunner.OpenTestFile));
+            PerformanceRunner.TimeAction(PerformanceRunner.OpenTestFile);
+            Console.WriteLine();
+
+            // Disable this block by default - I don't use it often
+#if false
+
+            Console.WriteLine("Running {0}", nameof(PerformanceRunner.RunInsertTable));
+            PerformanceRunner.TimeAction(PerformanceRunner.RunInsertTable);
+            Console.WriteLine();
+
+            Console.WriteLine("Running {0}", nameof(PerformanceRunner.PerformHeavyCalculation));
+            PerformanceRunner.TimeAction(PerformanceRunner.PerformHeavyCalculation);
+            Console.WriteLine();
+#endif
+
+            Console.WriteLine("Press any key to continue");
+            Console.ReadKey();
+        }
+    }
+}