support NODE_PATH in standalone build (#19617)

References https://github.com/tailwindlabs/tailwindcss/pull/19391.
References https://github.com/tailwindlabs/tailwindcss/pull/16274.

Right now, when using the standalone build of the TailwindCSS CLI, you
cannot use a custom `NODE_PATH`, but you can when using it via Node.js
directly.

A custom NODE_PATH allows you to resolve imports from multiple
locations. For example, in [Phoenix
LiveView](https://github.com/phoenixframework/phoenix_live_view/), we
have a feature where you can write scripts in templates that we extract
at compile time to a custom folder and users can import those in their
application bundle by saying

```javascript
import { hooks as colocatedHooks } from "phoenix-colocated/my_app"
```

where the "phoenix-colocated" folder lives in a different location than
the usual `node_modules` folder. This works fine with the default
esbuild setup, as it respects `NODE_PATH`, so we can pass it a custom
location.

We want to also support colocating CSS in templates soon, but the same
approach doesn't work with the standalone Tailwind CLI we ship with
default Phoenix projects. It works when running Tailwind through
Node.js, but we don't want to tell users they need to install it, just
to use the feature.

This patch changes the lookup logic for the standalone CLI to also
account for `NODE_PATH`. Note that you can pass multiple paths, that are
split according the the OS PATH separator.

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>