Feature Request: Make the builtin validators public for easier initialization

[JustinGrote]

Story

As a developer, I want a simpler way to initialize an option's validations using property initializers.

readonly Option<string> Script = new("--script", "-c")
{
  Description = "The PowerShell script to execute.",
  Validator = {
    FileOrDirectoryExists<FileInfo>
  }
};

Today I have to define it separately and then run Script.AcceptLegalFileNamesOnly() in a constructor, which is not ideal.

Solution

Change the validators such as FileOrDirectoryExists<T> https://github.com/dotnet/command-line-api/blob/702b2bbdd795ec978c3f2bb395a1a37442c13c8c/src/System.CommandLine/ArgumentValidation.cs to public so I can use them as shown above.