feat: added defaults functions for clean-css and csso

Author: cap-Bernardito

README.md

@@ -188,12 +188,19 @@ module.exports = {
 ### `minify`
 
 Type: `Function|Array<Function>`
-Default: `undefined`
+Default: `CssMinimizerPlugin.cssnanoMinify`
 
-Allows you to override default minify function.
+Allows to override default minify function.
 By default plugin uses [cssnano](https://github.com/cssnano/cssnano) package.
 Useful for using and testing unpublished versions or forks.
 
+Possible options:
+
+- CssMinimizerPlugin.cssnanoMinify
+- CssMinimizerPlugin.cssoMinify
+- CssMinimizerPlugin.cleanCssMinify
+- async (data, inputMap, minimizerOptions) => {return {code: `a{color: red}`, map: `...`, warnings: []}}
+
 > ⚠️ **Always use `require` inside `minify` function when `parallel` option enabled**.
 
 #### `Function`
@@ -206,36 +213,14 @@ module.exports = {
     minimize: true,
     minimizer: [
       new CssMinimizerPlugin({
-        minify: (data, inputMap, minimizerOptions) => {
-          const postcss = require('postcss');
-
-          const plugin = postcss.plugin(
-            'custom-plugin',
-            () => (css, result) => {
-              // custom code
-            }
-          );
-
-          const [[filename, input]] = Object.entries(data);
-
-          const postcssOptions = {
-            from: filename,
-            to: filename,
-            map: {
-              prev: inputMap,
+        minimizerOptions: {
+          level: {
+            1: {
+              roundingPrecision: 'all=3,px=5',
             },
-          };
-
-          return postcss([plugin])
-            .process(input, postcssOptions)
-            .then((result) => {
-              return {
-                code: result.css,
-                map: result.map,
-                warnings: result.warnings(),
-              };
-            });
+          },
         },
+        minify: CssMinimizerPlugin.cleanCssMinify,
       }),
     ],
   },
@@ -247,6 +232,37 @@ module.exports = {
 If an array of functions is passed to the `minify` option, the `minimizerOptions` must also be an array.
 The function index in the `minify` array corresponds to the options object with the same index in the `minimizerOptions` array.
 
+**webpack.config.js**
+
+```js
+module.exports = {
+  optimization: {
+    minimize: true,
+    minimizer: [
+      new CssMinimizerPlugin({
+        minimizerOptions: [
+          {}, // Options for the first function (CssMinimizerPlugin.cssnanoMinify)
+          {}, // Options for the second function (CssMinimizerPlugin.cleanCssMinify)
+          {}, // Options for the third function
+        ],
+        minify: [
+          CssMinimizerPlugin.cssnanoMinify,
+          CssMinimizerPlugin.cleanCssMinify,
+          async (data, inputMap, minimizerOptions) => {
+            // To do something
+            return {
+              code: `a{color: red}`,
+              map: `{"version": "3", ...}`,
+              warnings: [],
+            };
+          },
+        ],
+      }),
+    ],
+  },
+};
+```
+
 ### `minimizerOptions`
 
 Type: `Object|Array<Object>`