fix(plugin): use webpack 5 processAssets hook

Author: achipiga

lib/plugin.js

@@ -15,10 +15,18 @@ class Plugin {
 
   apply(compiler) {
     if (compiler.hooks) {
-      // setup hooks for webpack >= 4
-      compiler.hooks.compilation.tap('MangleCssClassPluginHooks', compilation => {
-        compilation.hooks.optimizeChunkAssets.tapAsync('MangleCssClassPluginOptimizeChunkAssetsHooks', runner(compiler, compilation, this.opts));
-      });
+      compiler.hooks.compilation.tap(
+        'MangleCssClassPluginHooks',
+        (compilation) => {
+          if (compilation.hooks.processAssets) {
+            // setup hooks for webpack >= 5
+            compilation.hooks.processAssets.tapAsync('MangleCssClassPluginOptimizeChunkAssetsHooks', runner(compiler, compilation, this.opts));
+          } else {
+            // setup hooks for webpack >= 4
+            compilation.hooks.optimizeChunkAssets.tapAsync('MangleCssClassPluginOptimizeChunkAssetsHooks', runner(compiler, compilation, this.opts));
+          }
+        }
+      );
     } else {
       // setup hooks for webpack <= 3
       compiler.plugin('compilation', (compilation) => {