Fixed %sapper.styles% injection for webpack apps

[artemjackson]

Hi again!

Several month ago I've added css field to build.json info file for webpack apps.
The css field had a shape like:

{
  main: string,
  chunks: Record<WebpackChunkName, CSSFileName>
}

That time I didn't notice two things:

1. Page part styles are retrieved via build_info.css[part.file]. Because of the implementation described above used the wepackChunkName to form ccs.chunks keys the styles were never added to %sapper.styles%.
2. The css.chunks values should be arrays not strings

The PR fixes both of the issues described.

[artemjackson]

Btw, I tested the PR against sapper@0.24.3.

Will it be possible to use the fix with sapper version lower the 0.26-alpha (to not to migrate to svelte 3)?

[Rich-Harris]

Sorry for the delay, Sapper has been a big neglected during the Svelte 3 release. I also published a 0.25.1 version with just this fix. Thanks!