fix getRelativePosition type definition

[aha-oretama]

I referred to this official document.
https://www.chartjs.org/docs/latest/configuration/interactions.html#converting-events-to-data-values

The argument of onClick function is defined here.

Chart.js/types/index.esm.d.ts

Line 1493 in 3792f9d

onClick(event: ChartEvent, elements: ActiveElement[], chart: Chart): void;

The getRelativePosition supports both types in JS code. The following getCanvasPosition function is called in getRelativePosition.

Chart.js/src/helpers/helpers.dom.js

Line 63 in bc7c58d

const e = evt.native || evt;

Then, the first argument of getRelativePosition should accept ChartEvent too.

[etimberg]

This is failing because ChartEvent is not defined. Once the CI passes, this looks good to me