Add support for Fluent Validation

[MrDave1999]

These methods can be distributed in a NuGet package:

public static class SRFluentValidationResultExtensions
{
    public static bool IsFailed(this ValidationResult result) => !result.IsValid;
    public static IEnumerable<string> AsErrors(this ValidationResult result)
        => result.Errors.Select(failure => failure.ErrorMessage);

   public static Result Invalid(this ValidationResult result) 
        => Result.Invalid(result.AsErrors());

   public static Result Invalid(this ValidationResult result, string message) 
        => Result.Invalid(message, result.AsErrors());
}

The SR "prefix" is so as not to cause name conflicts.

PD: The package was published here: https://www.nuget.org/packages/SimpleResults.FluentValidation