`dotnet run file.cs` `#:` directive for `ItemGroup`

### Is your feature request related to a problem? Please describe.

The [`dotnet run file.cs` document](https://github.com/dotnet/sdk/blob/main/documentation/general/dotnet-run-file.md) list the `#:property` directive to include a property (`PropertyGroup`) but no directive to include an item (`ItemGroup`).

### Describe the solution you'd like
- A `#:item name "value"` directive that generates
```xml



```

### Additional context
This would be useful to include additional files used by source-generators.
For example:

```c#
#:package Google.Protobuf 3.*
#:package Grpc.Tools 2.*

#:item Protobuf "somefile.proto"

Console.WriteLine("Do some stuff");
```