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/HelloWorld.cs

@@ -0,0 +1,16 @@
+using System;
+using ClosedXML.Excel;
+
+namespace ClosedXML_Examples
+{
+    public class HelloWorld
+    {
+        public void Create(String filePath)
+        {
+            var workbook = new XLWorkbook();
+            var worksheet = workbook.Worksheets.Add("Sample Sheet");
+            worksheet.Cell("A1").Value = "Hello World!";
+            workbook.SaveAs(filePath);
+        }
+    }
+}