Improve @tailwindcss/postcss performance for initial builds (#14565)

This PR improves the performance of the `@tailwindcss/postcss` plugin.
Before this change we created 2 compiler instances instead of a single
one. On a project where a `tailwindcss.config.ts` file is used, this
means that the timings look like this:

```
[@tailwindcss/postcss] Setup compiler: 137.525ms
⋮
[@tailwindcss/postcss] Setup compiler: 43.95ms
```

This means that with this small change, we can easily shave of ~50ms for
initial PostCSS builds.

---------

Co-authored-by: Philipp Spiess <hello@philippspiess.com>
Co-authored-by: Jordan Pittman <jordan@cryptica.me>

CHANGELOG.md

@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Use the right import base path when using the CLI to reading files from stdin ([#14522](https://github.com/tailwindlabs/tailwindcss/pull/14522))
 - Ensure that `@utility` is top-level and cannot be nested ([#14525](https://github.com/tailwindlabs/tailwindcss/pull/14525))
+- Only setup a single compiler in `@tailwindcss/postcss` for initial builds ([#14565](https://github.com/tailwindlabs/tailwindcss/pull/14565))
 - Ensure editing imported CSS files triggers a rebuild ([#14561](https://github.com/tailwindlabs/tailwindcss/pull/14561))
 - Ensure `@apply` and CSS functions work inside imported stylesheets ([#14576](https://github.com/tailwindlabs/tailwindcss/pull/14576))
 - _Experimental_: Improve codemod output, keep CSS after last Tailwind directive unlayered ([#14512](https://github.com/tailwindlabs/tailwindcss/pull/14512))