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_Tests/Excel/CalcEngine/CalcEngineCellEnumerationTests.cs

@@ -0,0 +1,23 @@
+using ClosedXML.Excel;
+using NUnit.Framework;
+
+namespace ClosedXML_Tests.Excel.CalcEngine
+{
+    public class CalcEngineCellEnumerationTests
+    {
+        [Test]
+        public void CanEnumerateCellsOverEmptySheet()
+        {
+            using (var wb = new XLWorkbook())
+            {
+                var sheet1 = wb.AddWorksheet("Sheet1");
+                var sheet2 = wb.AddWorksheet("Sheet2");
+
+                var cell = sheet1.FirstCell();
+                cell.FormulaA1 = "=SUMIFS(Sheet2!B:B, Sheet2!C:C, 1)";
+
+                Assert.AreEqual(0, cell.Value);
+            }
+        }
+    }
+}