Buildcheck template - grace handle names/dirs starting with numbers

[JanKrivanek]

Context

If msbuildcheck template is being instantiated with name (or implicit name derived from the current dir name) that starts with number - the include item fro the 'magic' props is broken

Repro steps

• dotnet new msbuildcheck -n 123
• The created csproj will contain broken include (the name _123.props doesn't correspond with the filename - 123.props):

  <ItemGroup>
    <None Include="_123.props" Pack="true" PackagePath="build\_123.props" />
    <Content Include="README.md" />
  </ItemGroup>

Analysis

This has to do with templating feature that is trying to make the names a valid C# token names - so numbers are auto-prefixed with '_'. We need to make sure that the include link is not expanded like this, or that the props file name is generated that way

[JanKrivanek]

Team triage: P2 - as we can ship template out of band

[JanKrivanek]

Can be documented in #10449