Add ValidationError constructor that accepts identifier and error message

Aloha!

There appears to be a good amount of overlap between FluentValidation's ValidationFailure and this project's ValidationError class. One slight difference is the ValidationFailure class has a constructor that accepts a property name and error message. Thoughts on adding something similar to the ValidationError class in this repository? I'm not suggesting modifying any of the current constructors, but simply adding one that accepts these two values:

```csharp
public ValidationError(string identifier, string errorMessage)
{
this.Identifier = identifier;
this.ErrorMessage = errorMessage;
}
```

This would simplify the following code:

```csharp
// Current state
new ValidationError(nameof(Model.Name), "Name is required.", "", ValidationSeverity.Error);
// Proposed enhancement
new ValidationError(nameof(Model.Name), "Name is required.")
```

If you approve of the above enhancement I'd be happy to open a PR! Cheers!

Locate the ValidationError class and inspect its existing constructors and properties, especially Identifier and ErrorMessage. Compare the current and proposed construction examples; done means the additional two-argument construction is supported without changing existing constructors and preserves the requested values.

Written by the indexing model from the issue text.