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/Utils/ExcelDocsComparer.cs

@@ -0,0 +1,38 @@
+using System;
+using System.IO;
+using System.IO.Packaging;
+
+namespace ClosedXML_Tests
+{
+    internal static class ExcelDocsComparer
+    {
+        public static bool Compare(string left, string right, out string message)
+        {
+            using (FileStream leftStream = File.OpenRead(left))
+            using (FileStream rightStream = File.OpenRead(right))
+            {
+                return Compare(leftStream, rightStream, out message);
+            }
+        }
+
+        public static bool Compare(Stream left, Stream right, out string message)
+        {
+            using (Package leftPackage = Package.Open(left, FileMode.Open, FileAccess.Read))
+            using (Package rightPackage = Package.Open(right, FileMode.Open, FileAccess.Read))
+            {
+                return PackageHelper.Compare(leftPackage, rightPackage, false, ExcludeMethod, out message);
+            }
+        }
+
+        private static bool ExcludeMethod(Uri uri)
+        {
+            //Exclude service data
+            if (uri.OriginalString.EndsWith(".rels") ||
+                uri.OriginalString.EndsWith(".psmdcp"))
+            {
+                return true;
+            }
+            return false;
+        }
+    }
+}