Add editor for n-star ratings, backed by an integer value

★★★★★

Number of stars are determined by `maximum` (default: `5`) and optionally `exclusiveMaximum` (default: `false`).

````javascript
rating: {
  type: "integer",
  format: "rating",
  maximum: "5",
  exclusiveMaximum: false,
  readonly: false
}
````