Consider differentiating these two `*CompletionContext` scenarios using fewer public types

[jonsequitur]

    public class TextCompletionContext : CompletionContext 
    {
        public string CommandLineText { get; }
        public int CursorPosition { get; }
        public TextCompletionContext AtCursorPosition(int position);
    }
 
    // Does this class need to be public?  (Could some other library instantiate one meaningfully?)
    public class TokenCompletionContext : CompletionContext 
    {
    }