Proj0019: Order using directives alphabetically

<Using> nodes should be ordered alphabetically within a single <ItemGroup> in order to make it more human readable. When ordering the references based on other criteria, consider grouping them in seperate <ItemGroup>s.

Non-compliant

<Project Sdk="Microsoft.NET.Sdk">

  <ItemGroup>
    <Using Include="NamespaceB" />
    <Using Include="NamespaceA" />
  </ItemGroup>
  
</Project>

Compliant

<Project Sdk="Microsoft.NET.Sdk">

  <ItemGroup>
    <Using Include="NamespaceA" />
    <Using Include="NamespaceB" />
  </ItemGroup>
  
</Project>