feat: webpack plugin doesn't need all chunk info (#735)

I'm currently working on a pretty massive project with hundreds of split points, thousands of modules (plus node_modules), CSS, hot reloading, all the goodies... but *LoadablePlugin* was taking a very significant amount of time to run (about 1.2s). With these changes it went down to ~7ms.

packages/webpack-plugin/src/index.js

@@ -22,7 +22,7 @@ class LoadablePlugin {
     const stats = compilation.getStats().toJson({
       all: false,
       assets: true,
-      chunks: true,
+      chunks: false,
       chunkGroups: true,
       chunkGroupChildren: true,
       hash: true,
@@ -34,11 +34,12 @@ class LoadablePlugin {
     stats.generator = 'loadable-components'
 
     // we don't need all chunk information, only a type
-    stats.chunks = stats.chunks.map(chunk => ({
-      ...chunk,
-      modules: [], // in case modules array is big
-      origins: [], // in case origins array is big
-    }))
+    stats.chunks = [...compilation.chunks].map((chunk) => {
+      return {
+        id: chunk.id,
+        files: [...chunk.files],
+      };
+    });
 
     const result = JSON.stringify(stats, null, 2)