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/Examples/CommentsTests.cs

@@ -0,0 +1,15 @@
+using ClosedXML_Examples;
+using NUnit.Framework;
+
+namespace ClosedXML_Tests.Examples
+{
+    [TestFixture]
+    public class CommentsTests
+    {
+        [Test]
+        public void AddingComments()
+        {
+            TestHelper.RunTestExample<AddingComments>(@"Comments\AddingComments.xlsx");
+        }
+    }
+}