Add support for 'slnx'

[dansiegel]

What would you like to be added:

Add support for the slnx format.

<Solution>
  <Folder Name="/Solution Items/">
    <File Path=".gitignore" />
    <File Path="Directory.Build.props" />
    <File Path="Directory.Build.targets" />
    <File Path="Directory.Packages.props" />
    <File Path="global.json" />
  </Folder>
  <Project Path="UnoApp1\UnoApp1.csproj" Type="C#">
    <Deploy />
  </Project>
  <Properties Name="Visual Studio">
    <Property Name="OpenWith" Value="17" />
  </Properties>
</Solution>

Why is this needed:

This is an updated solution format that is ultimately easier to maintain than the traditional solution format. It's also likely that this can remain part of the template even when being created in Visual Studio.

[spouliot]

Note that MS C# extension does not yet support .slnx for VS Code.
See microsoft/vscode-dotnettools#1533