feat: add stat object that stores transformation information

[GoodBoyDigital]

Added some stats and a few other bits!

1. add stats object to assets

   • All assets not have a little stat object that stores transformation information. Currently this is transform time, transform date, success and error (if success is false).

2. expose root asset

   • Added a rootAsset to AssetPack so devs who are feeling adventurous can gain access to the underlying assets

3. tweak how we use the cache

   • Whilst writing the test, i realised that when calling run on the same assetpack instance multiple times and in between runs, changing the assets in the input folder then things get broken!
   • The main reason for this was that the cache was written correctly between runs, but only read on the first run. I have adjusted the API so that now AssetWatcher now expects an AssetCache (with read and write functions) to be passed to it. Seemed to do the trick!

As a follow up I would like to create a generate stats function that makes a little report or data dump for users, but that can come later - this is good as a foundational PR!