Declares a static method of a VisualElementComponentAttribute struct as the handler that runs when one of the component's fields changes on an element.
The method must be @@static void M(ref T self, VisualElement owner), and a
component may declare at most one. The source
generator emits notifying SetXxx(ref T, VisualElement, value) setters for the component's
fields; each one flags the owner so this handler runs once in the next update, no matter how many
fields changed. Use it to invalidate the owner — for example owner.MarkDirtyRepaint()@@, a
layout request, or recomputing a cached value. The handler never runs from inside the setter, so a
burst of writes in one frame coalesces into a single call.