Closes#14253
Since we changed the export strategy for the postcss client in #14132,
we accidentally no longer generated type exports for this package.
This PR adds a type export back. We now use a similar pattern to the
`./colors` and `./defaultTheme` exports in the tailwindcss package where
we have a separate cjs entrypoint.
The changes were validated manually in a playground project that were
installing the updated dependencies from tarballs.
Here is one example of it working as expected:
<img width="750" alt="Screenshot 2024-08-26 at 14 10 07"
src="https://github.com/user-attachments/assets/83de15f2-1543-4805-9231-9b8df1636c5e">
