package.json: properly configure xo and postcss

xo doesn't understand CSS syntax so it was useless that it was checking CSS files.

Also, configure autoprefixer so that its changes do not need a prettier fix after that.

Signed-off-by: XhmikosR <xhmikosr@gmail.com>

postcss.config.js

@@ -0,0 +1,11 @@
+/* eslint-env node */
+
+"use strict";
+
+module.exports = (/* ctx */) => ({
+  plugins: {
+    autoprefixer: {
+      cascade: false,
+    },
+  },
+});