build: Add DotNet.ReproducibleBuilds package

- Added a new package reference for DotNet.ReproducibleBuilds in the Directory.Build.props file.
- Additionally, three files (`README.md`, `LICENSE`, and `icon.png`) are now included in the PackageInfoFiles item group of the Directory.Build.props file.
- The icon.png file has been added.
- Finally, the version.json file has been updated to use "4.0-alpha" as the version instead of "4.0-beta".

Directory.Build.props

@@ -11,4 +11,15 @@
     <PackageReference Include="Nerdbank.GitVersioning" Version="3.6.133" Condition="!Exists('packages.config')" PrivateAssets="all" >
     </PackageReference>
   </ItemGroup>
+
+  <ItemGroup Condition="$(PackAsTool) != 'true'">
+    <PackageReference Include="DotNet.ReproducibleBuilds" Version="1.2.4" PrivateAssets="All"/>
+  </ItemGroup>
+
+  <ItemGroup Label="PackageInfoFiles">
+    <!-- Import the Readme at build props level if there is none in the project -->
+    <None Condition="!Exists('README.md')" Include="$(_DirectoryBuildPropsBasePath)\README.md" Pack="true" PackagePath="/" />
+    <None Include="$(MSBuildThisFileDirectory)\LICENSE" Pack="true" PackagePath="/" />
+    <None Include="$(MSBuildThisFileDirectory)\icon.png" Pack="true" PackagePath="/" />
+  </ItemGroup>
 </Project>

version.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/master/src/NerdBank.GitVersioning/version.schema.json",
-  "version": "4.0-beta",
+  "version": "4.0-alpha",
   "publicReleaseRefSpec": [
     "^refs/heads/master$",
     "^refs/heads/v\\d+(?:\\.\\d+)?$"