Use the modern Sass API

[nex3]

Parcel currently uses the deprecated legacy API for compiling Sass code. This API is less efficient and less correct than the modern API, and is slated to be removed in an upcoming Dart Sass 2.0.0 release. In order to encourage users to migrate away before that, we'll be adding a deprecation warning to all uses of the legacy API in one of the next few Sass releases.

Also consider adding support for the sass-embedded package, and particularly the Compiler API. When doing multiple small compilations, this API on the embedded host is much faster than the plain-JS sass package.