Reproduction link or steps

Try migrating any of the https://github.com/netlify/primitives/ packages.

1. Clone repo
2. go to e.g. packages/types
3. npx tsdown migrate
4. ENTER
5. check new config

What is expected?

The splitting option is not in the config (as it doesn't exist in tsdown), but depending on the value, other options are set to reflect the desired behavior.

What is actually happening?

The option is still there.

Any additional comments?

No response