TypeScript doesn't know about import usages in the template because it only sees the script of a Svelte file. Therefore preserve all value imports. Requires TS 4.5 or higher, therefore also bump the version of TS and Prettier (the latter in order to deal with the new TS syntax).
Turning on preserveValueImports turns off the advanced but somewhat brittle import transpilation in svelte-preprocess which should give a small performance boost and more robustness.