WithSeverity - support for errors severity level

Feature description

Add severity support to error output.
- I'm not sure if it's needed... Maybe it's bringing to much complexity into something that should be fairly simple.
Severities could be replaced with... multiple validators.

Specification<string> specification = s => s
    .NotEmpty().WithSeverity(10)
    .NotWhitespace().WithSeverity(0)

Specification<string> specification = s => s
    .NotEmpty().WithSeverity(Severity.High)
    .NotWhitespace().WithSeverity(Severity.Low)

result.AnyErrorsWithSeverity(Severity.High);

result.AnyErrorsWithSeverity(10);

result.GetMessageMapWithSeverity(Severity.Medium | Severity.High)

New parameter command: WithSeverity.
Would be marking the entire error output with a severity level.
Result would have methods to filter out error outputs that have certain level of severity...