sv migrate svelte-5 should update reactive use of Map to SvelteMap

[eedmond]

Due to Svelte 5's change that classes are no longer auto reactive, the migration tool should not create $state(new Map(...)), but instead $state(new SvelteMap(...)).

It took me a long time to sort out why my component was no longer reactive after the update, so this would likely save people a lot of headache. Thanks!

SvelteMap docs