[BUG] `panzoomchange` event is not understood in Svelte

The library we are using sends updates through a custom `panzoomchange` event, which we listen on.

Svelte does not know what this event is, and is giving errors in the editor. In a future version of Svelte, this might also block linting.

We should fix this, how exactly I am not sure. There should be a way to register an event such that Svelte understands what it is.