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/Misc/HyperlinkTests.cs

@@ -0,0 +1,32 @@
+using ClosedXML.Excel;
+using NUnit.Framework;
+
+namespace ClosedXML_Tests.Excel.Misc
+{
+    [TestFixture]
+    public class HyperlinkTests
+    {
+        [Test]
+        public void TestHyperlinks()
+        {
+            using (var wb = new XLWorkbook())
+            {
+                var ws1 = wb.Worksheets.Add("Sheet1");
+                var ws2 = wb.Worksheets.Add("Sheet2");
+
+                var targetCell = ws2.Cell("A1");
+                var targetRange = ws2.Range("A1", "B1");
+
+                var linkCell1 = ws1.Cell("A1");
+                linkCell1.Value = "Link to IXLCell";
+                linkCell1.Hyperlink = new XLHyperlink(targetCell);
+                Assert.AreEqual("Sheet2!A1", linkCell1.Hyperlink.InternalAddress);
+
+                var linkRange1 = ws1.Cell("A2");
+                linkRange1.Value = "Link to IXLRangeBase";
+                linkRange1.Hyperlink = new XLHyperlink(targetRange);
+                Assert.AreEqual("Sheet2!A1:B1", linkRange1.Hyperlink.InternalAddress);
+            }
+        }
+    }
+}