fix: avoid accidentally overriding sass config with scss configs (#4637)

Author: haoqunjiang

packages/@vue/cli-service/__tests__/css.spec.js

@@ -347,7 +347,8 @@ test('scss loaderOptions', () => {
             prependData: sassData
           },
           scss: {
-            prependData: scssData
+            prependData: scssData,
+            webpackImporter: false
           }
         }
       }
@@ -365,6 +366,9 @@ test('scss loaderOptions', () => {
     },
     sourceMap: false
   })
+
+  // should not merge scss options into default sass config
+  expect(findOptions(config, 'sass', 'sass')).not.toHaveProperty('webpackImporter')
 })
 
 test('should use dart sass implementation whenever possible', () => {

packages/@vue/cli-service/lib/config/css.js

@@ -187,11 +187,13 @@ module.exports = (api, rootOptions) => {
     createCSSRule('css', /\.css$/)
     createCSSRule('postcss', /\.p(ost)?css$/)
     createCSSRule('scss', /\.scss$/, 'sass-loader', Object.assign(
+      {},
       defaultSassLoaderOptions,
       loaderOptions.scss || loaderOptions.sass
     ))
     if (sassLoaderVersion < 8) {
       createCSSRule('sass', /\.sass$/, 'sass-loader', Object.assign(
+        {},
         defaultSassLoaderOptions,
         {
           indentedSyntax: true
@@ -200,6 +202,7 @@ module.exports = (api, rootOptions) => {
       ))
     } else {
       createCSSRule('sass', /\.sass$/, 'sass-loader', Object.assign(
+        {},
         defaultSassLoaderOptions,
         loaderOptions.sass,
         {