fix(nextjs): specify return type in withNx plugin

[G07cha]

Current Behavior

Inherited type doesn't exactly match NextConfig causing type-errors when used in combination with other plugins

Expected Behavior

withNx returns valid NextConfig

Related Issue(s)

None

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✅ Successfully ran 12 targets

• nx-cloud record -- yarn depcheck
• nx affected --target=lint --base=65a079cbe134541e541847ab7b25c8ddcbca2a97 --head=0f3da38a6ff0aa4bea67ee744f3a73eaf1b1667b --parallel=3
• nx affected --target=e2e --base=65a079cbe134541e541847ab7b25c8ddcbca2a97 --head=0f3da38a6ff0aa4bea67ee744f3a73eaf1b1667b --exclude=e2e-storybook,e2e-storybook-angular,e2e-react-native,e2e-detox,e2e-make-angular-cli-faster --parallel=1
• nx-cloud record -- yarn nx workspace-lint
• nx-cloud record -- yarn check-lock-files
• nx affected --target=build --base=65a079cbe134541e541847ab7b25c8ddcbca2a97 --head=0f3da38a6ff0aa4bea67ee744f3a73eaf1b1667b --parallel=3
• nx affected --target=test --base=65a079cbe134541e541847ab7b25c8ddcbca2a97 --head=0f3da38a6ff0aa4bea67ee744f3a73eaf1b1667b --parallel=1
• nx-cloud record -- yarn check-commit
• nx-cloud record -- yarn nx format:check --base=65a079cbe134541e541847ab7b25c8ddcbca2a97 --head=0f3da38a6ff0aa4bea67ee744f3a73eaf1b1667b
• nx-cloud record -- yarn check-imports
• nx-cloud record -- yarn documentation
• nx build typedoc-theme

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1 Ignored Deployment

Name	Status	Preview	Updated
nx-dev	⬜️ Ignored (Inspect)		Oct 10, 2022 at 9:44AM (UTC)

[jaysoo]

Looks good, thanks

[github-actions]

This pull request has already been merged/closed. If you experience issues related to these changes, please open a new issue referencing this pull request.