Split SDK (Rider plugin)

Adds a .csproj quick fix that replace the SDK line in the file header with two import statements.

Turns this:

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

</Project>

into this

<Project>
    <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" />
    <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />
</Project>

This is useful if you want to add code after the Sdk.targets line.