feat(v2): introduce new minification of CSS bundle (#3716)

* feat(v2): optimize CSS bundle

* Move to separate preset

* Move custom scrollbar styles to separate class

* Cleanup styles

* Remove unactual styles

* Various CSS optimizations, cleanup styles for NProgress

* Add ability to back to old way of minifying CSS

* chore(v2): downgrade babel-plugin-dynamic-import-node to 2.3.0

* Use env var for back to simple CSS minifier

* remove unnecessary typing [skip-ci]

* Remove missing dep

* Update website/docs/cli.md

Co-authored-by: Sébastien Lorber <slorber@users.noreply.github.com>

packages/docusaurus-cssnano-preset/src/remove-overridden-custom-properties/index.js

@@ -0,0 +1,42 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const postcss = require('postcss');
+
+/*
+This PostCSS plugin will remove duplicate/same custom properties (which are actually overridden ones) **only** from `:root` selector.
+
+Depending on the presence of an `!important` rule in value of custom property, the following actions will happens:
+
+- If the same custom properties do **not** have an `!important` rule, then all of them will be removed except for the last one (which will actually be applied).
+- If the same custom properties have at least one `!important` rule, then only those properties that do not have this rule will be removed.
+*/
+
+module.exports = postcss.plugin(
+  'postcss-remove-overridden-custom-properties',
+  () => {
+    return (root) => {
+      root.walkDecls((decl) => {
+        if (decl.parent.selector !== ':root') {
+          return;
+        }
+
+        const sameProperties =
+          decl.parent.nodes.filter((n) => n.prop === decl.prop) || [];
+        const hasImportantProperties = sameProperties.some((p) =>
+          p.hasOwnProperty('important'),
+        );
+
+        const overriddenProperties = hasImportantProperties
+          ? sameProperties.filter((p) => !p.hasOwnProperty('important'))
+          : sameProperties.slice(0, -1);
+
+        overriddenProperties.map((p) => p.remove());
+      });
+    };
+  },
+);