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';

[nex3]

I'm not sure this is a desirable change. Moving from invoking the executable function to invoking the JS API causes a number of issues, including a substantial amount of new code we need to maintain and much higher friction synchronizing this into Google (where we use the Dart executable instead). It also bakes in one particular importer scheme rather than giving users the flexibility to define custom importers—although that would also add a considerable amount of complexity.

I think the principled way to address this use-case is with sass/sass#2739, which would provide a consistent syntax for package imports that wouldn't need to be maintained ad hoc in a Bazel package.