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

consistent with vue-cli

Author: PanJiaChen

.postcssrc.js

@@ -108,7 +108,6 @@
   },
   "browserslist": [
     "> 1%",
-    "last 2 versions",
-    "not ie <= 8"
+    "last 2 versions"
   ]
 }

package.json

@@ -0,0 +1,5 @@
+module.exports = {
+  plugins: {
+    autoprefixer: {}
+  }
+}