Add postcss as a dependency of @tailwindcss/postcss (#14750)

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`.

---

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.

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Allow spaces spaces around operators in attribute selector variants ([#14703](https://github.com/tailwindlabs/tailwindcss/pull/14703))
 - Ensure color opacity modifiers work with OKLCH colors ([#14741](https://github.com/tailwindlabs/tailwindcss/pull/14741))
 - Ensure changes to the input CSS file result in a full rebuild ([#14744](https://github.com/tailwindlabs/tailwindcss/pull/14744))
+- Add `postcss` as a dependency of `@tailwindcss/postcss` ([#14750](https://github.com/tailwindlabs/tailwindcss/pull/14750))
 - _Upgrade (experimental)_: Migrate `flex-grow` to `grow` and `flex-shrink` to `shrink` ([#14721](https://github.com/tailwindlabs/tailwindcss/pull/14721))
 - _Upgrade (experimental)_: Minify arbitrary values when printing candidates ([#14720](https://github.com/tailwindlabs/tailwindcss/pull/14720))
 - _Upgrade (experimental)_: Ensure legacy theme values ending in `1` (like `theme(spacing.1)`) are correctly migrated to custom properties ([#14724](https://github.com/tailwindlabs/tailwindcss/pull/14724))

packages/@tailwindcss-postcss/package.json

@@ -34,13 +34,13 @@
     "@tailwindcss/node": "workspace:^",
     "@tailwindcss/oxide": "workspace:^",
     "lightningcss": "catalog:",
+    "postcss": "^8.4.41",
     "tailwindcss": "workspace:^"
   },
   "devDependencies": {
     "@types/node": "catalog:",
     "@types/postcss-import": "14.0.3",
     "internal-example-plugin": "workspace:*",
-    "postcss": "^8.4.41",
     "postcss-import": "^16.1.0"
   }
 }