Turning the "importsNotUsedAsValues" TS config value to "error".
This will require us to use `import type` instead of `import` when we are importing a value that is only used as a type (and therefore that is dropped by the Typescript compiler).
Why this change?
This is a requirement to be able to use Svelte in the future. See https://github.com/sveltejs/svelte-preprocess/issues/206#issuecomment-663193798
