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/Styles/StyleFill.cs

@@ -0,0 +1,27 @@
+using ClosedXML.Excel;
+using System;
+
+namespace ClosedXML_Examples.Styles
+{
+    public class StyleFill : IXLExample
+    {
+        public void Create(String filePath)
+        {
+            var workbook = new XLWorkbook();
+            var ws = workbook.Worksheets.Add("Style Fill");
+
+            var co = 2;
+            var ro = 1;
+
+            ws.Cell(++ro, co + 1).Value = "BackgroundColor = Red";
+            ws.Cell(ro, co).Style.Fill.BackgroundColor = XLColor.Red;
+
+            ws.Cell(++ro, co + 1).Value = "PatternType = DarkTrellis; PatternColor = Orange; BackgroundColor = Blue";
+            ws.Cell(ro, co).Style.Fill.PatternType = XLFillPatternValues.DarkTrellis;
+            ws.Cell(ro, co).Style.Fill.PatternColor = XLColor.Orange;
+            ws.Cell(ro, co).Style.Fill.BackgroundColor = XLColor.Blue;
+
+            workbook.SaveAs(filePath);
+        }
+    }
+}