fix(nextjs): Update Next.js page generator to support app/ router

[ndcunningham]

If you have a Next.js project that uses app/ router, when you run the page generator

npx nx generate @nx/next:page about --project=my-project --no-interactive --dry-run 

it should generate something similar to this

>  NX  Generating @nx/next:page

CREATE app/about/page.module.css
CREATE app/about/page.tsx

closes: #17352

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Name	Status	Preview	Comments	Updated (UTC)
nx-dev	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 2, 2023 6:21pm

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✅ Successfully ran 8 targets

• nx affected -t e2e-macos --parallel=1 --base=8f771e023b40992f8a99487561cec481297f8aff --head=6d53832f9034b3a780acb5843fcbb7916e507916
• nx affected --target=e2e --base=8f771e023b40992f8a99487561cec481297f8aff --head=6d53832f9034b3a780acb5843fcbb7916e507916 --parallel=1
• nx affected --target=build --base=8f771e023b40992f8a99487561cec481297f8aff --head=6d53832f9034b3a780acb5843fcbb7916e507916 --parallel=3
• nx affected --target=test --base=8f771e023b40992f8a99487561cec481297f8aff --head=6d53832f9034b3a780acb5843fcbb7916e507916 --parallel=1
• nx run-many -t check-imports check-commit check-lock-files depcheck check-codeowners documentation --parallel=1 --no-dte
• nx affected --target=lint --base=8f771e023b40992f8a99487561cec481297f8aff --head=6d53832f9034b3a780acb5843fcbb7916e507916 --parallel=3
• nx-cloud record -- nx format:check --base=8f771e023b40992f8a99487561cec481297f8aff --head=6d53832f9034b3a780acb5843fcbb7916e507916
• nx documentation --no-dte

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