Reference examples for @svelte/legacy migration

[esbeto]

Would be nice to have reference implementation examples for newcomers or people unfamiliar with the helper event modifiers that were deprecated in Svelte 5.

In the migration guide we have an example of how to migrate preventDefault.
https://svelte.dev/docs/svelte/v5-migration-guide#Migration-script-Event-modifiers

But some helpers are a bit more involved, like once, passive, self. It would be easier to migrate if we have similar examples for others.