Enhance typings for .scss files

[fxlrnrpt]

Currently, we use a global type declaration

declare module '*.scss' {
  const classes: { [key: string]: string }
  export default classes
}

We need to proper types generated for the .scss files. Ideally, we should not generate appropriate .d.ts files, but somehow help the TS compiler process .scss files directly (research TSC addon/extension functionality).