Somehow only recheck changed files in svelte-check watch mode

[dummdidumm]

Right now svelte-check naively reruns all diagnostics on all files on change. For Svelte and CSS checks we can change that to only rerun on the affected files because we know that these diagnostics or per-file and don't have dependencies on other files. TS/JS type checks not so much, there theoretically the whole program can be affected by a change. Still, there might be some (hidden?) way/method inside the TS LS that runs diagnostics on the whole program in a more efficient way.

Edit: We already cache the compiler output of Svelte files, so there probably isn't much perf win there.