Add support for single validation error with Result.Invalid

[IliyanAng]

More often than not I find myself passing a single Validation error in the Result.Invalid method. This PR addresses that. I do believe this to be a needed improvement, as it's very annoying to create new Lists only to pass a single object in them.

• Support for single validation error in the Result.Invalid method
• Added unit tests to validate behavior
• All existing behavior remains the same. The single instance of the Validation Error will be passed in a new List upon Result.Invalid creation

   //This instantiates a new list with the single validation error in it
   public new static Result Invalid(ValidationError validationError)
      {
          return new Result(ResultStatus.Invalid) { ValidationErrors = { validationError } };
      }

[ardalis]

Love it and agree, thanks!

[github-actions[bot]]

Package	Line Rate	Branch Rate	Complexity	Health
Ardalis.Result.FluentValidation	86%	50%	6	✔
Ardalis.Result.Sample.Core	18%	18%	63	❌
Ardalis.Result	23%	7%	73	❌
Summary	24% (93 / 387)	19% (10 / 54)	142	❌