Declares a static method of a VisualElementComponentAttribute struct that runs once, right after the component is added to an element.
The method must be @@static void M(ref T self, VisualElement owner), and a
component may declare at most one. It runs on every add path: AddComponent,
GetOrAddComponent@@, a component authored in UXML, and a
[RequiresComponentOfType] auto-add. It runs
after the component is stored and its [RegisterCallback] handlers are registered, so the
method sees a fully wired component and may configure the owner element (for example set
owner.focusable). On the UXML path it runs when the component is created, before the
authored attribute values are applied, so authored values always win over values this method
writes to the component.