feat: Add support for webpack style import from node_modules.

[lukasholzer]

To enable the support I had to switch to the JavaScript API https://sass-lang.com/documentation/js-api#rendersync as it is already orchestrated via javascript. This enables the possibility of specifying importers that can resolve custom import paths.

Fixes #98

[lukasholzer]

Sorry for the formatting changes but my buildifier formatted it. 😬

@jelbourn any opinions on how to add node_modules with a scss dependency only for the examples or tests to showcase the webpack flavour import:

@import '~@angular/cdk/overlay';