Adds an event handler that runs when an element's font value is changed.
An element triggers a change event when a site visitor changes the font value in the panel element.
A change event is not triggered when you change an element's font value using
the element's fontFamily property in code.
Because onFontChange() is fired before validations are performed, do not check
any of the validation properties in the onFontChange() event handler. For example,
do not code an if statement that checks the valid property in onFontChange()
because the property at this point contains values prior to the onFontChange() event.
handler(event: Event): void The name of the function or
the function expression to run when the element's value changes.