Handle loading Sapper rollup config

That exports 3 configs

```
{
client,
server,
serviceWorker
}
```

Example https://github.com/andrewdc/cy-unit-test/blob/master/rollup.config.js

Start by comparing the referenced rollup.config.js with the preprocessor's current configuration-loading entry point. Verify how a configuration exporting client, server, and serviceWorker is handled, and consider the work complete when that Sapper-style configuration loads correctly without breaking existing Cypress Rollup behavior.

Written by the indexing model from the issue text.