Sort of, via preprocessing, but you won't get any real benefit from it at present. There are two barriers: the compiler itself needs to become TS-aware, and we need to teach editors like VSCode how to typecheck inside <script> blocks.

It's very much on the radar (I'm a TS convert, personally — Svelte itself is written in TypeScript), we just need to get round to it. The version 3 rewrite was largely about laying the foundations for TS support.