Using the svelte field in package.json to point at .svelte source files is deprecated and you must use a svelte export condition. vite-plugin-svelte 3 still resolves it as a fallback, but in a future major release this is going to be removed and without exports condition resolving the library is going to fail.
https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/faq.md#missing-exports-condition
Using the svelte field in package.json to point at .svelte source files is deprecated and you must use a svelte export condition. vite-plugin-svelte 3 still resolves it as a fallback, but in a future major release this is going to be removed and without exports condition resolving the library is going to fail.