Replace cssnano with CSSO

CSSO has better PostCSS API, has less dependencies and produces more
optimized CSS.

package.json

@@ -47,7 +47,6 @@
   "devDependencies": {
     "@size-limit/preset-small-lib": "^7.0.5",
     "autoprefixer": "^10.4.2",
-    "cssnano": "^5.0.16",
     "eslint": "^8.8.0",
     "eslint-config-prettier": "^8.3.0",
     "eslint-plugin-unicorn": "^40.1.0",
@@ -56,6 +55,7 @@
     "postcss-calc": "^8.2.3",
     "postcss-cli": "^9.1.0",
     "postcss-css-variables": "^0.18.0",
+    "postcss-csso": "^6.0.0",
     "prettier": "^2.5.1",
     "size-limit": "^7.0.5",
     "vite": "^2.8.0"

postcss.config.js

@@ -8,10 +8,8 @@ module.exports = {
   plugins: [
     require("postcss-css-variables"),
     require("postcss-calc"),
-    require("cssnano")({
-      preset: "default",
-    }),
     require("autoprefixer"),
+    require("postcss-csso"),
     require("postcss-banner")({
       banner: bannerText,
       important: true,