feat(Result): Expose value or error if it is save to expose

[GregOnNet]

This PR proposes two Type-Guards, allowing to safely access error or value of a Result.
This should improve the Ergonomics of the Result-API.

hasValue

// instead of
if (result.isSuccess) {
  const value = result.getValueOrThrow();
}

// do
if (result.hasValue()) {
  const value = result.value; // value is accessible because the type-guard has been executed before
}

hasError

// instead of
if (result.isFailure) {
  const error = result.getErrorOrThrow();
}

// do
if (result.hasError()) {
  const error= result.error; // erroris accessible because the type-guard has been executed before
}

Some notes

• I needed to introduce two new Methods because Type-Guards are not usable with Getters (reference: Custom type guard in getters is broken microsoft/TypeScript#7150 (comment))
• Changing the existing getters isSuccess and isFailure to methods would introduce a breaking change.
• Personally, I would love to use isSuccess() directly. Maybe this could be a breaking change for the next major release.

[seangwright]

Thanks for the contribution! I'll get a new release pushed out soon.