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/Program.cs

@@ -0,0 +1,34 @@
+using System;
+using System.IO;
+
+namespace ClosedXML_Examples
+{
+    public class Program
+    {
+        public static string BaseCreatedDirectory
+        {
+            get
+            {
+                var path = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Created");
+                if (!Directory.Exists(path)) Directory.CreateDirectory(path);
+                return path;
+            }
+        }
+
+        public static string BaseModifiedDirectory
+        {
+            get
+            {
+                var path = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Modified");
+                if (!Directory.Exists(path)) Directory.CreateDirectory(path);
+                return path;
+            }
+        }
+
+        private static void Main(string[] args)
+        {
+            CreateFiles.CreateAllFiles();
+            LoadFiles.LoadAllFiles();
+        }
+    }
+}