Update type for webpack's Stats.ToJsonOutput['assetsByChunkName'] (#42219)

`assetsByChunkName` is an object keyed by chunk name with a value of either a single asset (string) or array of assets.

See implementation here: https://github.com/webpack/webpack/blob/v4.41.5/lib/Stats.js#L441-L447

types/webpack/index.d.ts

@@ -1583,7 +1583,7 @@ declare namespace webpack {
                 name: string;
                 size: number;
             }>;
-            assetsByChunkName?: Record<string, Record<string, string[]>>;
+            assetsByChunkName?: Record<string, string | string[]>;
             builtAt?: number;
             children?: Array<ToJsonOutput & { name?: string }>;
             chunks?: Array<{