- Updated `rollData` in helper.js to use actor as the data source rather
than the object. This allows for items to reference attributes by
assuming the parent actor is the source of truth and item attributes
would be referenced as either `@item.attr` or `@items.ITEMNAME.attr`
- Refactored formula replacement to prevent values from being replaced
with 0s unintentionally.
- Refactored actor-sheet.js to move most of its logic into a separate
helper.js module as static methods on a new SimpleHelper class. This
doesn't add anything functional at the moment, but it's in
preparation of re-using the same code for the item-sheet.js file.
