Adds an event handler that runs when the element is clicked.
An element receives a click event when a user clicks on the element
and releases.
When a user double-clicks an element, two click events are fired
before a doubleClick event is also fired.
Note: To avoid unpredictable behavior, don't set redirect links in the editor for elements that already have an onClick event handler.
handler(event: MouseEvent): void The name of the function or
the function expression to run when the element is clicked.