Proj1703: MSBuild child elements should be separated by an empty line

For readabillity purposes, MSBuild child elements (such as <PropertyGroup>, <ItemGroup>, and <Choose>) should be separated by an empty line. <Import> elements should be grouped without an empty line between them.

Non-compliant

<Project Sdk="Microsoft.NET.Sdk">
  <Import Project="shared/first.props" />
  <Import Project="shared/second.props" />
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
  </PropertyGroup>
  <ItemGroup Label="Project under test">
    <ProjectReference Include="../../src/DotNetProjectFile.Analyzers/DotNetProjectFile.Analyzers.csproj" />
  </ItemGroup>
  <ItemGroup Label="Test tools">
    <PackageReference Include="AwesomeAssertions" />
    <PackageReference Include="CodeAnalysis.TestTools" />
    <PackageReference Include="NUnit" />
  </ItemGroup>
  <ItemGroup Label="Build tools">
    <PackageReference PrivateAssets="all" Include="coverlet.collector" />
    <PackageReference PrivateAssets="all" Include="Microsoft.Build.Tasks.Core" />
    <PackageReference PrivateAssets="all" Include="Microsoft.NET.Test.Sdk" />
    <PackageReference PrivateAssets="all" Include="NUnit3TestAdapter" />
  </ItemGroup>
</Project>

Compliant

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

  <Import Project="shared/first.props" />
  <Import Project="shared/second.props" />
  
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
  </PropertyGroup>
  
  <ItemGroup Label="Project under test">
    <ProjectReference Include="../../src/DotNetProjectFile.Analyzers/DotNetProjectFile.Analyzers.csproj" />
  </ItemGroup>
  
  <ItemGroup Label="Test tools">
    <PackageReference Include="AwesomeAssertions" />
    <PackageReference Include="CodeAnalysis.TestTools" />
    <PackageReference Include="NUnit" />
  </ItemGroup>
  
  <ItemGroup Label="Build tools">
    <PackageReference PrivateAssets="all" Include="coverlet.collector" />
    <PackageReference PrivateAssets="all" Include="Microsoft.Build.Tasks.Core" />
    <PackageReference PrivateAssets="all" Include="Microsoft.NET.Test.Sdk" />
    <PackageReference PrivateAssets="all" Include="NUnit3TestAdapter" />
  </ItemGroup>

</Project>