Improve story for adding on sass support to webapp preset

@ecraig12345 2019-02-21 01:42:18发表于 microsoft/just Needs: Design :thought_balloon:

Currently if you want to build sass files, you can't just use the webapp task preset, and multiple manual config modifications are required (build, webpack, jest). We should provide an easier way to handle that.

There are two types of scenarios we may want to support for building/bundling sass files:

  1. Webpack: Convert and bundle the files using webpack loaders (some combination of sass-loader, postcss-loader, autoprefixer, css-loader, @microsoft/loader-load-themed-styles). Might still need separate handling for jest.
  2. AMD-compatible: Convert the sass files to ts files first, then load/bundle with AMD (what fabric does today)

We're probably more interested in doing (1) in a reusable manner, since most people are moving away from AMD.