Brojd/plugin fixes (#3947)

## Which issue(s) this PR fixes
- use correct webpack merge strategy
- 
## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

grafana-plugin/webpack.config.ts

@@ -46,10 +46,10 @@ const config = async (env): Promise<Configuration> => {
         use: CustomizeRule.Merge,
       },
     },
-    plugins: CustomizeRule.Replace,
     watchOptions: {
       use: CustomizeRule.Merge,
     },
+    plugins: CustomizeRule.Merge,
   })(baseConfig, customConfig);
 };