Preprocess options should be on options.preprocess

[Rich-Harris]

I just realised that preprocess functions are being drawn directly from options:

options: {
  style: (...) => {...}
}

For consistency with other integrations, that should be like this instead:

options: {
  preprocess: {
    style: (...) => {...}
  }
}

We'll have to intercept options.style, options.script and options.markup and a) turn them into options.preprocess and b) print a big deprecation warning.