Add support for dotnet file.cs (without explicit run subcommand)

[jjonescz]

No description provided.

[vrubleg]

Please make sure that .csx file extension is also allowed. The .csx file extension was used for C# scripts for ages. It was possible to execute those by csi.exe from the MSBuild distribution, and dotnet run looks like a great way to move forward.

Having a separate extension makes clear that it's the entry point and can be executed directly. Some people might even register .csx to be executed directly without prepending dotnet run (that is possible to do with shebang on Linux).