drop support for svelte.config.cjs

[dominikg]

Describe the problem

the current code for loading svelte.config still allows svelte.config.cjs to be used. This is a bit convoluted and the original need for adding svelte.config.cjs support (jest compatibility) is largely gone thanks to vitest.

Describe the proposed solution

simplify load config by removing all cjs related code, going forward config must be esm and named svelte.config.js or svelte.config.mjs

Alternatives considered

Dropping it immediately with the current pre-release could be a bit short notice for the few projects that still have a svelte.config.cjs.

We can add a deprecation notice log and remove it in the next major after.

Importance

would make my life easier

[dominikg]

If you are a current user of svelte.config.cjs and have a continued need for it, please share here.