fix(nextjs): Move @sentry/webpack-plugin back to non-dev dependencies (#3981)

This reverts the change made in #3866. The SDK user needs the webpack plugin in order to upload their sourcemaps, so it needs to be installed when they install the SDK.

Author: lobsterkatie

packages/nextjs/package.json

@@ -23,11 +23,11 @@
     "@sentry/react": "6.12.0",
     "@sentry/tracing": "6.12.0",
     "@sentry/utils": "6.12.0",
+    "@sentry/webpack-plugin": "1.17.1",
     "tslib": "^1.9.3"
   },
   "devDependencies": {
     "@sentry/types": "6.12.0",
-    "@sentry/webpack-plugin": "1.17.1",
     "@types/webpack": "^5.28.0",
     "eslint": "7.20.0",
     "next": "10.1.3",