fix: allow svelte.config.cjs

[forrestjacobs]

Addresses #35:

• Update Cosmiconfig to v7, which allows "svelte.config.cjs"
• Search for "svelte.config.cjs" as well as "svelte.config.js" when preprocess is true.

I don't love the test I added since it depends on the code that svelte generates, but I didn't want to make it too complex. Let me know if you want me to make changes. I'm not very available for the next 48 hours so if you want to make changes yourself or if someone else opens a better PR, I'm not too precious about my contribution -- I just selfishly want to be able to use svelte.config.cjs myself!