Is your feature request related to a problem? Please describe.
This modifier seems to have been ignored or forgotten.
I have used Svelte to create a TV application on Samsungs Tizen platform.
I needed to use stopImmediatePropagation all the time there, because the TV OS had its own set of events which fired some DOM events causing callbacks to be executed twice.

Describe the solution you'd like
I would like for a stopImmediatePropagation modifier to be added to the project which would work exactly as on:click|stopPropagation.

How important is this feature to you?
It's not ultra important but adds consistency to handling events in Svelte.