Give elements their own interface definition

From what I understand, if we want to allow registering additional
element options (see [here][1] for an example), then the element options
need to be a top-level interface so that they can be used with
TypeScript's [declaration merging][2].

[1]: chartjs/chartjs-plugin-annotation#275 (comment)
[2]: https://www.typescriptlang.org/docs/handbook/declaration-merging.html

types/index.esm.d.ts

@@ -1834,14 +1834,16 @@ export const BarElement: ChartComponent & {
 	new (cfg: any): BarElement;
 };
 
+export interface ElementOptions {
+	arc: ArcOptions & ArcHoverOptions;
+	bar: BarOptions & BarHoverOptions;
+	line: LineOptions & LineHoverOptions;
+	point: PointOptions & PointHoverOptions;
+}
 export interface ElementChartOptions {
-	elements: {
-		arc: ArcOptions & ArcHoverOptions;
-		bar: BarOptions & BarHoverOptions;
-		line: LineOptions & LineHoverOptions;
-		point: PointOptions & PointHoverOptions;
-	};
+	elements: ElementOptions;
 }
+
 export class BasePlatform {
 	/**
 	 * Called at chart construction time, returns a context2d instance implementing