Consider a `CliAction` design that doesn't require custom implementations to override both `Invoke` and `InvokeAsync` methods

[jonsequitur]

public abstract class CliAction
{
    public abstract int Invoke(ParseResult parseResult);
    public abstract Task<int> InvokeAsync(ParseResult parseResult, CancellationToken cancellationToken = default);
}

[KalleOlaviNiemitalo]

ICommandHandler was replaced with CliAction in #2095, but this issue applies to that too.