Reproduction link or steps

Add a subpackage export on Windows laptop enabling export: true in the tsdown.config.ts file:

What is expected?

Subpackage exports should use / instead separator path

What is actually happening?

Using separator path, requires applying a normalization

Any additional comments?

I'm going to send a PR to fix this bug.

In fact the generated subpackages exports are wrong: we need to handle multiple entries and group them: check core/types and core/index, maybe a side effect of this issue (?)