Add: success and failure events

[rg-wood]

Fire events on a success or failure. Success is determined by min or max attributes:

<vellum-dice min="7">3d6+3</vellum-dice>

Success or failure changes the colour of the component text. Defaults are green for success, red for failure. We will create --success-color and --failure-color CSS variables to customise these defaults.

On a successful roll, the component emits a success event. Conversely, failed rolls emit failure events. This can be scripted as follows:

const dice = document.querySelector('vellum-dice')

function success(event) {
  alerts(`success on ${event.detail.result}`
}

dice.addEventListener('success', success)

Notes:

• What should be the behaviour if both min and max attributes are set?
• Should we support handling other kinds of success (i.e. rolling doubles)?