As suggested by @AlexJ
The event you passed through $event is a DOM event, therefore you can use the EventName as the type.
In your case this event is a MouseEvent, and the docs says, quoting
The MouseEvent interface represents events that occur due to the user interacting with a pointing device (such as a mouse). Common events using this interface include click, dblclick, mouseup, mousedown.
In all those cases you’ll get a MouseEvent.
Another example : if you have this code
<input type="text" (blur)="event($event)"
When the event triggers you’ll get a FocusEvent.
So you can do it really simple, console log the event and you’ll see a message similar to this one that’ll we have the event name
FocusEvent {isTrusted: true, relatedTarget: null, view: Window, detail: 0, which: 0…}
You can always visit the docs for a list of existing Events.
Edit
You can also check for TypeScript dom.generated.d.ts with all the typings ported. In your case stopPropagation() is part of Event, extended by MouseEvent.