Add postcss as a dependency of @tailwindcss/postcss

[RobinMalfait]

This PR adds postcss as a dependency of @tailwindcss/postcss.

If you are in an environment with Next.js where you can use the @tailwindcss/postcss package, then postcss is required.

If you have postcss in your package.json, then everything is fine, however if you don't then you will get an error that postcss cannot be found.

Note: this only happens when you are using npm, if you are using pnpm, then the postcss package can be resolved correctly and you won't run into issues. This is also why the integration tests just worked (because we use pnpm).

To make sure that this package works for most people in most environments, we explicitly add postcss as a dependency of @tailwindcss/postcss.

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I wanted to create an integration test for this to make sure this works, but we are currently using pnpm with some of pnpm's features to make sure that we can override dependencies that point to .tgz files.

[RobinMalfait]

• Add postcss as a dependency of @tailwindcss/postcss #14750 👈
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