[DOM] Copy `source` onto the synthetic toggle event (#37389)

`ToggleEvent` carries a `source` property pointing at the control that
opened or closed a popover, but `ToggleEventInterface` only lists
`newState` and `oldState`, so the synthetic event never copies it. An
`onToggle` handler reads `event.source` as `undefined` even when the
native event has it.

Adding `source` to the interface copies it off the native event the same
way `newState` and `oldState` are copied.