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/ExampleHelper.cs

@@ -0,0 +1,19 @@
+using System.IO;
+
+namespace ClosedXML_Examples
+{
+    public static class ExampleHelper
+    {
+        public static string GetTempFilePath()
+        {
+            return Path.GetTempFileName();
+        }
+
+        public static string GetTempFilePath(string filePath)
+        {
+            var extension = Path.GetExtension(filePath);
+            var tempFilePath = GetTempFilePath();
+            return Path.ChangeExtension(tempFilePath, extension);
+        }
+    }
+}