Add extension methods to the ModelStateDictionary type

[MrDave1999]

These extension methods will be added to the ModelStateDictionary type:

public static class SRModelStateDictionaryExtensions
{
    public static bool IsFailed(this ModelStateDictionary modelState);
    public static Result Invalid(this ModelStateDictionary modelState);
    public static Result Invalid(this ModelStateDictionary modelState, string message);
    public static BadRequestObjectResult BadRequest(this ModelStateDictionary modelState);
    public static BadRequestObjectResult BadRequest(this ModelStateDictionary modelState, string message);
}

SRModelStateDictionaryExtensions contains methods that convert an instance of ModelStateDictionary into an instance of type Result.
The SR "prefix" is so as not to cause name conflicts.

Usage

[Route("api/[controller]")]
public class PersonController : ControllerBase
{
    [HttpPost]
    public ActionResult<Result<CreatedId>> Create([FromBody]Person person)
    {
        if (ModelState.IsFailed())
            return ModelState.BadRequest();

        // Some code..
    }
}