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/Misc/AddingDataTableAsWorksheet.cs

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+using System;
+using System.Data;
+using System.Linq;
+using ClosedXML.Excel;
+
+namespace ClosedXML_Examples.Misc
+{
+    public class AddingDataTableAsWorksheet : IXLExample
+    {
+        #region Variables
+
+        // Public
+
+        // Private
+
+
+        #endregion
+
+        #region Properties
+
+        // Public
+
+        // Private
+
+        // Override
+
+
+        #endregion
+
+        #region Events
+
+        // Public
+
+        // Private
+
+        // Override
+
+
+        #endregion
+
+        #region Methods
+
+        // Public
+        public void Create(String filePath)
+        {
+            var wb = new XLWorkbook();
+
+            var dataTable = GetTable("Information");
+
+            // Add a DataTable as a worksheet
+            wb.Worksheets.Add(dataTable);
+            wb.Worksheets.First().Columns().AdjustToContents();
+
+            wb.SaveAs(filePath);
+        }
+
+        // Private
+        private DataTable GetTable(String tableName)
+        {
+            DataTable table = new DataTable();
+            table.TableName = tableName;
+            table.Columns.Add("Dosage", typeof(int));
+            table.Columns.Add("Drug", typeof(string));
+            table.Columns.Add("Patient", typeof(string));
+            table.Columns.Add("Date", typeof(DateTime));
+
+            table.Rows.Add(25, "Indocin", "David", new DateTime(2000, 1, 1));
+            table.Rows.Add(50, "Enebrel", "Sam", new DateTime(2000, 1, 2));
+            table.Rows.Add(10, "Hydralazine", "Christoff", new DateTime(2000, 1, 3));
+            table.Rows.Add(21, "Combivent", "Janet", new DateTime(2000, 1, 4));
+            table.Rows.Add(100, "Dilantin", "Melanie", new DateTime(2000, 1, 5));
+            return table;
+        }
+        // Override
+
+
+        #endregion
+    }
+}