Add better Transition debugging information.

[rwjblue]

Expose a few nice debugging tools when working with a transition:

interface Transition {
  /**
    In non-production builds, this function will return the stack that this Transition was
    created within. In production builds, this function will not be present.

    @method debugCreationStack
    @return string
  */
  debugCreationStack?: () => string | undefined;

  /**
    In non-production builds, this function will return the stack that this Transition was
    aborted within (or `undefined` if the Transition has not been aborted yet). In production
    builds, this function will not be present.

    @method debugAbortStack
    @return string
  */
  debugAbortStack?: () => string | undefined;

  /**
    In non-production builds, this property references the Transition that _this_ Transition
    was derived from or `undefined` if this transition did not derive from another. In
    production builds, this property will not be present.

    @property debugPreviousTransition
    @type {Transition | undefined}
  */
  debugPreviousTransition: Maybe<Transition<T>>;
}

---

Note: I've confirmed that Ember's build pipeline properly replaces the
if (DEBUG) { bits with true and false as appropriate:

if (true
    /* DEBUG */
) {
  var _error = new Error(`Transition creation stack`);

  this.debugCreationStack = () => _error.stack;

  this.debugPreviousTransition = previousTransition;
}

[rwjblue]

FWIW, this only affects the local tests (not what consumers use). Ember's build pipeline (as well as general Ember apps) uses nearly the same thing...