feat: propagate item pointermove and click events

[silvester-pari]

Implemented changes

This PR listens for the item pointermove and item click events inside of the Vega chart and propagates them as pointermove.item and click.item events. They do not trigger on anything else on the chart (axis, title etc.) - one can use the native pointermove or click event for that.

It can be tested in the storybook by observing the console. In the bar chart example, it correctly logs the hovered and clicked bar, and if hovered/clicked above, it logs the clicked GroupItem.

Screenshots/Videos

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• I have added a test related to this feature/fix
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[StefanBrand]

In general looks good. I've added some comments around naming of the events and testing.