FYI: LegalFilePathsOnly extension method is ambiguous in C# 7.2 and lower

I got a C# compiler error like this, in a project consuming the System.CommandLine package:

> error CS0121: The call is ambiguous between the following methods or properties: 'ArgumentExtensions.LegalFilePathsOnly\(TArgument)' and 'OptionExtensions.LegalFilePathsOnly\(TOption)'

I was able to fix that by upgrading my project from C# 7 to C# 7.3. I presume this corresponds to [What's new in C# 7.3](https://docs.microsoft.com/dotnet/csharp/whats-new/csharp-7-3) item "Overload resolution now has fewer ambiguous cases."

Filing this in case anyone else has the same problem.

To reproduce

Visual Studio 2017 version 15.9.24 and .NET Core SDK 2.1.515.

Source code:

```C#
using System.CommandLine;

namespace Demo
{
internal static class Program
{
private static void Main()
{
new Option("file").LegalFilePathsOnly();
}
}
}
```

Project:

```XML

Exe
netcoreapp2.1
7

```