Webpack fails to resolve scss files imported by other scss files in node_modules

[ethanroday]

Steps to reproduce:

1. Run preact create <app-name>
2. cd <app-name>
3. npm install --D material-web-components
4. In a .scss file, include @import "~@material/drawer/mdc-drawer"; to import the styles for the Material drawer.
5. npm run build

Expected result:

The build will succeed.

Actual result:

The build fails. It turns out that @material/drawer/mdc-drawer is dependent on @material/rtl/mixins but, since that import isn't prefixed by a ~, Webpack doesn't know where to find it.
A quick fix (although probably not the most efficient fix) is to just tell sass-loader (and less-loader) to look in node_modules to resolve imports. I will do a quick PR if that seems like the right solution.