Extends `FormController` to provide label activation via form associated custom elements via a shim where not supported (currently just Safari).
Performing label activation is the responsibility of form associated elements.
Helpers are provided to facilitate this, including `isActivationClick` and `dispatchActivationClick`. Note, any element that should perform an action via a click, could use these helpers to help distinguish external "activation" clicks.
PiperOrigin-RevId: 495689270
