Add details regarding piping C# via stdin to `dotnet run` to file-based apps reference

[DamianEdwards]

Describe the issue or suggestion

We should update the new file-based apps reference (https://github.com/dotnet/docs/blob/main/docs/core/sdk/file-based-apps.md) to include details of the support for piping C# directly to dotnet run - via standard in. The - argument on dotnet run indicates that it should read the code from stdin, e.g.:

PowerShell

'Console.WriteLine("hello from stdin!");' | dotnet run -

Bash

echo 'Console.WriteLine("hello from stdin!");' | dotnet run -

@meaghanlewis @BillWagner

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Associated WorkItem - 540970