make sveltekit look more idiomatic

[oetiker]

Using the $app/env we can make this look much more idiomatic. While still ensuring the code only runs in the browser.

The compiler is not entirely happy as it still complains:

svelte-carousel doesn't appear to be written in CJS, but also doesn't appear to be a valid ES module (i.e. it doesn't have "type": "module" or an .mjs extension for the entry point). Please contact the package author to fix.

but the resulting code seems to work fine