Add toggleable rollUnder for Special Abilities

[orffen]

This will be passed to successChatMessage as data-roll-under for determining success/failure.

Need migration to add a rollUnder: true to existing Item.type === 'feature'

[orffen]

Think this can be easily handled in item.mjs under prepareData(), something like:

const itemData = this;

if (this.type === 'feature') {
  this.system.rollUnder.value = this.system.rollUnder.value ?? true;
}

But before I do that I want to confirm what happens when template.json is updated; because FoundryVTT does have an internal migration process of some kind.

[orffen]

Good hack before we move to a system data model.

[orffen]

Added under 001cc47ba38963e3f8c80ecbace1e29932fa65f7