fix: use createRequire to require svelte.config.cjs in esm project

[bluwy]

Fixes #141

Import .cjs config file if app is running in type: module.

Also added a few more tests for loading different config files.

[bluwy]

Initially went with the import("svelte.config.cjs") strategy, which surprisingly works, but apparently the timestamp query cache bust doesn't actually work for cjs only and keeps returning the same reference when updating the svelte config. Luckily the CI caught this.

So I changed to using createRequire instead if we're in esm (checked via import.meta.url). And that seems to work without too much overhead.

[dominikg]

I couldn't find docs that mention import.meta.url is only set when it's running in esm mode, but it makes sense. Thanks!