Terminal output color settings for webpack stats (#31583)

Webpack stats support terminal color definition string as such:

```json
colors: {
  green: '\u001b[32m',
},
```

types/webpack/index.d.ts

@@ -15,6 +15,7 @@
 //                 John Reilly <https://github.com/johnnyreilly>
 //                 Ryan Waskiewicz <https://github.com/rwaskiewicz>
 //                 Kyle Uehlein <https://github.com/kuehlein>
+//                 Grgur Grisogono <https://github.com/grgur>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -1228,7 +1229,7 @@ declare namespace webpack {
 
         interface ToStringOptionsObject extends ToJsonOptionsObject {
             /** `webpack --colors` equivalent */
-            colors?: boolean;
+            colors?: boolean | string;
         }
 
         type ToStringOptions = Preset | ToStringOptionsObject;