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/XmlEncoderTests.cs

@@ -0,0 +1,23 @@
+using ClosedXML.Utils;
+using NUnit.Framework;
+
+namespace ClosedXML_Tests.Excel
+{
+    [TestFixture]
+    public class XmlEncoderTest
+    {
+        [Test]
+        public void TestControlChars()
+        {
+            Assert.AreEqual("_x0001_ _x0002_ _x0003_ _x0004_", XmlEncoder.EncodeString("\u0001 \u0002 \u0003 \u0004"));
+            Assert.AreEqual("_x0005_ _x0006_ _x0007_ _x0008_", XmlEncoder.EncodeString("\u0005 \u0006 \u0007 \u0008"));
+
+            Assert.AreEqual("\u0001 \u0002 \u0003 \u0004", XmlEncoder.DecodeString("_x0001_ _x0002_ _x0003_ _x0004_"));
+            Assert.AreEqual("\u0005 \u0006 \u0007 \u0008", XmlEncoder.DecodeString("_x0005_ _x0006_ _x0007_ _x0008_"));
+            Assert.AreEqual("\uAABB \uAABB", XmlEncoder.DecodeString("_xaaBB_ _xAAbb_"));
+
+            // https://github.com/ClosedXML/ClosedXML/issues/1154
+            Assert.AreEqual("_Xceed_Something", XmlEncoder.DecodeString("_Xceed_Something"));
+        }
+    }
+}