Improve storySeed initialisation

The storySeed now takes the timestamp directly and the initial seed is
stored as the storySeed.

It also no longer uses modulo 100, which should give a lot more
variety.

src/engine/StoryState.ts

@@ -350,9 +350,9 @@ export class StoryState {
     this._turnIndices = new Map();
     this.currentTurnIndex = -1;
 
-    let timeSeed = new Date().getTime();
-    this.storySeed = new PRNG(timeSeed).next() % 100;
-    this.previousRandom = 0;
+    let timeSeed = Date.now();
+    this.storySeed = new PRNG(timeSeed).nextSeed();
+    this.previousRandom = this.storySeed;
 
     this._currentChoices = [];