fix: remove DeprecationWarning from webpack4

Author: githoniel

src/index.js

@@ -187,17 +187,20 @@ export default class CSSSplitWebpackPlugin {
       // Run on `emit` when user specifies the compiler phase
       // Due to the incorrect css split + optimization behavior
       // Expected: css split should happen after optimization
-      compiler.plugin('emit', (compilation, done) => {
+      compiler.hooks.emit.tapAsync('css-split-emit', (compilation, done) => {
         return this.chunksMapping(compilation, compilation.chunks, done);
       });
     } else {
       // Only run on `this-compilation` to avoid injecting the plugin into
       // sub-compilers as happens when using the `extract-text-webpack-plugin`.
-      compiler.plugin('this-compilation', (compilation) => {
-        compilation.plugin('optimize-chunk-assets', (chunks, done) => {
-          return this.chunksMapping(compilation, chunks, done);
+      compiler.hooks.thisCompilation.tap('css-split-this-compilation',
+        (compilation) => {
+          compilation.hooks.optimizeChunkAssets.tapAsync(
+            'css-split-optimize-chunk-assets',
+            (chunks, done) => {
+              return this.chunksMapping(compilation, chunks, done);
+            });
         });
-      });
     }
   }
 }