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/OpenXMLTests.cs

@@ -0,0 +1,30 @@
+using DocumentFormat.OpenXml.Packaging;
+using NUnit.Framework;
+using System.IO;
+
+namespace ClosedXML_Tests
+{
+    [TestFixture]
+    public class OpenXMLTests
+    {
+        [Test]
+        [Ignore("Workaround has been included in ClosedXML")]
+        public static void SetPackagePropertiesEntryToNullWithOpenXml()
+        {
+            // Fixed in .NET Standard 2.1
+            // See:
+            //      https://github.com/OfficeDev/Open-XML-SDK/issues/235
+            //      https://github.com/dotnet/corefx/issues/23795
+            using (var stream = TestHelper.GetStreamFromResource(TestHelper.GetResourcePath(@"Examples\PivotTables\PivotTables.xlsx")))
+            using (var ms = new MemoryStream())
+            {
+                stream.CopyTo(ms);
+
+                using (var document = SpreadsheetDocument.Open(ms, true))
+                {
+                    document.PackageProperties.Creator = null;
+                }
+            }
+        }
+    }
+}