don't optimize svelte/ssr

### Describe the bug

svelte/ssr is only used on ssr build, so doesn't need to be optimized. As older versions of svelte don't export it, it can lead to an error where vite can't optimize the missing export 

### Reproduction

https://github.com/sveltejs/kit/issues/2181

### Logs

_No response_

### System Info

```shell
.
```


### Severity

blocking an upgrade