feat!: upgrade css-loader to v2 (#3866)

BREAKING CHANGE:
See https://github.com/webpack-contrib/css-loader/blob/bc16c3db953dbf4d711753fbb0cc60253def6916/CHANGELOG.md#200-2018-12-07

Author: haoqunjiang

packages/@vue/cli-service/package.json

@@ -41,7 +41,7 @@
     "clipboardy": "^2.0.0",
     "cliui": "^5.0.0",
     "copy-webpack-plugin": "^4.6.0",
-    "css-loader": "^1.0.1",
+    "css-loader": "^2.1.1",
     "cssnano": "^4.1.10",
     "current-script-polyfill": "^1.0.0",
     "debug": "^4.1.1",