Adds an event handler that runs when the value of a widget property is changed.
A widget receives an onPropsChanged event when one or more of its properties is changed via its API.
handler(oldProps: object, newProps: object): void The name of the function or
the function expression to run when the value of a widget property is changed.
This method doesn’t return any custom errors, but may return standard errors. Learn more about standard Wix errors.