Proj0021: Build actions should have a single task

<ItemGroup> build taks nodes such as <Compile>, <Content>, <EmbeddedResource>, <None>, and <AdditionalFiles should only have on task. Assigning multiple tasks to a single node separated by semicolons (;) leads to longer lines that are harder to read and maintain.

Non-compliant

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

  <ItemGroup>
    <None Include="../common/LICENSE.md;README.md" />
  </ItemGroup>
  
</Project>

Compliant

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

  <ItemGroup>
    <None Include="../common/LICENSE.md" />
    <None Include="README.md" />
  </ItemGroup>
</Project>