It seems that the official way to add typescript to svelte is to implement a LSP(from https://github.com/sveltejs/svelte/issues/4518).

I’m not a team member of the svelte project so I have no control over the decision. I have already suggested this approach(svelte-draft) in https://github.com/sveltejs/svelte/issues/3677#issuecomment-580226362, but I didn’t receive a single comment from the core team member.