A button's onClick can now be async. If a Promise is returned, the

button will be in a busy state until the Promise is resolved. This

ensures that the button can't be clicked a second time, and means the

caller doesn't have to micro-manage this.
Handlers that do need to micro-manage the state (such as setting the

busy state for multiple buttons at once) simply don't need to be async.

The button will work as before.
The Ink stories have been updated to show off click handlers for both

cases.

Tested both async and standard onClick handlers.