How do I add Sass compilation in Angular CLI 6: angular.json?

Doing exactly what Smokey Dawson commented,

"projects": {
  "angular-app": {
    "root": "",
    "sourceRoot": "src",
    "projectType": "application",
    "prefix": "app",
    "schematics": {
      "@schematics/angular:component": {
        "styleext": "scss"
      }
    },
    "architect": {...}
  }
}

The above is the result.
So in your app, under the key schematics,
add a key @schematics/angular:component
with the key styleext set to scss.

This should be applied to your angular.json file in the root directory of your project.