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
In particular, this allows external build tools like Rollup and @web/dev-server to understand imports of Lit, which is configured to require the ".js" extension via its export conditions (so that Lit import maps can remain minimal).
PiperOrigin-RevId: 469772992
