fix: @font-face inside :global block

[bfanger]

Fixes #236

<style lang="scss">
:global {
  @font-face {
    font-family: "Open Sans";
    src: url("/fonts/OpenSans-Regular.woff2") format("woff2"),
  }
}
</style>

Currently generates:

@font-face {}

Instead of the desired:

@font-face {
  font-family: "Open Sans";
  src: url("/fonts/OpenSans-Regular.woff2") format("woff2"),
}

"you shouldn't need to use :global for @font-face" was the reason for closing #236
And while this is an ok solution for when the @font-face is written inside the svelte component, I've encountered this issue when using the following pattern from the Readme:

<style lang="scss">
  :global {
    @import 'global-stylesheet.scss';
  }
}

All the @font-face rules in global-stylesheet.scss are currently being emptied by svelte-preprocess.
This PR fixes that