Add some decorators

[balloob]

Somehow we are not enforcing prettier, allowing the button PR to slip through unprettified. Those changes are included in this PR.

Decorators is a stage 2 draft proposal. It allows decorating classes, properties and methods and enhance them. It has been included in TypeScript under an experimental flag.

Lit Element ships with a bunch of decorators that make writing elements even easier:

• @customElement("hui-entity-button-card") to register a class as a custom element.
• @property(options) to register a field as a property. That way we don't have to define a properties method. (btw, we should only mark fields as properties if we want changing them to trigger a render)
• @query('.header') or @queryAll('.card') to turn a property into a getter that returns the results of querySelector or querySelectorAll on the shadow root

  @query('.header')
  protected headerElement!: HTMLDivElement;

• Bonus decorator: material web components have an observer decorator: @observer((value, old) => { … }) . The observer method will be called whenever the value has changed and the DOM has been rendered (during the updated lifecycle).