Fix init option (initial animation) type (#942)

* Update options.d.ts

Update "Initial animation" option (init) type.
- https://www.chartjs.org/chartjs-plugin-annotation/latest/guide/configuration.html#initial-animation

* Fix init option (initial animation) return type

AnnotationElement might be Partial<>

types/options.d.ts

@@ -43,7 +43,7 @@ export interface CoreAnnotationOptions extends AnnotationEvents, ShadowOptions,
   display?: Scriptable<boolean, PartialEventContext>,
   drawTime?: Scriptable<DrawTime, PartialEventContext>,
   hitTolerance?: Scriptable<number, PartialEventContext>,
-  init?: boolean | ((chart: Chart, properties: AnnotationBoxModel, options: AnnotationOptions) => void | boolean | AnnotationElement),
+  init?: boolean | (({ chart: Chart, properties: AnnotationBoxModel, options: AnnotationOptions }) => void | boolean | Partial<AnnotationElement>),
   id?: string,
   xMax?: Scriptable<ScaleValue, PartialEventContext>,
   xMin?: Scriptable<ScaleValue, PartialEventContext>,