Added Catch2 library for native unit tests.

Source/ThirdParty/catch2/LICENSE.txt

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+Boost Software License - Version 1.0 - August 17th, 2003
+
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
+works are solely in the form of machine-executable object code generated by
+a source language processor.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
+FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.

Source/ThirdParty/catch2/catch2.Build.cs

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+// Copyright (c) 2012-2021 Wojciech Figat. All rights reserved.
+
+using System.Collections.Generic;
+using System.IO;
+using Flax.Build;
+
+/// <summary>
+/// https://github.com/catchorg/Catch2
+/// </summary>
+public class catch2 : HeaderOnlyModule
+{
+    /// <inheritdoc />
+    public override void Init()
+    {
+        base.Init();
+
+        LicenseType = LicenseTypes.BoostSoftwareLicense;
+        LicenseFilePath = "LICENSE.txt";
+
+        // Merge into test binary
+        BinaryModuleName = "Engine.Tests";
+    }
+
+    /// <inheritdoc />
+    public override void GetFilesToDeploy(List<string> files)
+    {
+        base.GetFilesToDeploy(files);
+
+        files.AddRange(Directory.GetFiles(FolderPath, "*.hpp", SearchOption.AllDirectories));
+    }
+}