feat: enable property decorator for property accessors

[pskelin]

It is now possible to use the @property decorator on a property accessor like this:

class Accessor extends UI5Element {
  storage: boolean = false;

  @property()
  set myProp(value: boolean) {
    this.storage = value;
    // do sync work here
  }

  get myProp() {
    return this.storage;
  }

  render() {
    return html`<div>${this.myProp}</html>`;
  }
}

The main use case for this is to to synchronously make a DOM operation (like adding a class to show the dialog when a property is set), as the normal setting of a property triggers an async rendering cycle. A real use case is setting the focus to the first element of the dialog will show the iOS keyboard only if it is in the same tick as the click handler that triggered the operation (#8583)

Implementation

Previously, the @property decorator was overwriting such accessors and using the internal UI5Element _state for storage. With this change, if an accessor pair exists, it will be used for storage, this triggering the custom logic.

Additionally, setting the property will still trigger invalidation, as well as update the attribute in the DOM.