refactor[chore]: generate postcss.config.js instead of .postcssrc.js

consistent with vue-cli

.postcssrc.js

-// https://github.com/michael-ciniawsky/postcss-load-config
-
-module.exports = {
-  "plugins": {
-    // to edit target browsers: use "browserslist" field in package.json
-    "autoprefixer": {}
-  }
-}

package.json

@@ -107,7 +107,6 @@
   },
   "browserslist": [
     "> 1%",
-    "last 2 versions",
-    "not ie <= 8"
+    "last 2 versions"
   ]
 }

postcss.config.js

+module.exports = {
+  plugins: {
+    autoprefixer: {}
+  }
+}