chore: add scss to LoaderOptions interface (#7039)

Author: hiblacker

packages/@vue/cli-service/types/ProjectOptions.d.ts

@@ -13,6 +13,7 @@ interface PageConfig {
 interface LoaderOptions {
   css?: object;
   sass?: object;
+  scss?: object;
   less?: object;
   stylus?: object;
   postcss?: object;