Improving type-safety for the failed matches

[evilguest]

Hello. The current representation of the MatchResult relies on the user properly checking for the failed()/succeeded() before accessing the message/shortMessage properties.
With the current typescript we could achieve a substantial improvement at almost no cost. Idea is to move the failure-related properties to the FailedMatchResult that extends the regular MatchResult, remove the nullability marker, and change the result type of the failed() method to the type guard:

export interface MatchResult {
  succeeded(): boolean;
  failed(): this is FailedMatchResult;
}
export interface FailedMatchResult extends MatchResult
{
  message: string;   // defined
  shortMessage: string; /defined
  getInterval(): Interval;
}

This change is backwards-compatible for the code that did perform the failed() check:

if(match.failed()) // within then branch the match does have the FailedMatchResult type
{
  console.log(match.message); // safe to acces
}

The code broken by this change would be easy to fix by adding the corresponding guard.

[pdubroy]

Good idea! I've pulled this into an issue in #515.