Include app directory in Tailwind config

[phantaaa]

Updated with-tailwindcss example to include new app directory according to Tailwind docs

Bug

• Related issues linked using fixes #number
• Integration tests added
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Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
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Documentation / Examples

• Make sure the linting passes by running pnpm build && pnpm lint
• The "examples guidelines" are followed from our contributing doc

[balazsorban44]

This example currently does not have an app/ directory or is not even enabled via next.config.js. I wonder if this change makes sense as of now.

[phantaaa]

For people that want to make use of Next.js 13 app directory yes, it took me a while at first to figure out why my Tailwind doesn't work when adapting to new changes. Official Tailwind docs already added it so doesn't hurt to add it here as well.