Add `--stream` flag to `canonicalize` subcommand (#19796)

## Summary

- Adds `--stream` flag to `tailwindcss canonicalize` that reads
candidate groups from stdin line by line and writes canonicalized
results to stdout
- Keeps the design system loaded across requests, making it suitable as
a long-running sidecar process
- Empty lines pass through, keeping request/response pairs aligned

## Motivation

Non-JS tools (formatters, editor plugins, etc.) currently have no
lightweight way to canonicalize Tailwind classes. The existing batch
mode works for one-off use, but tools that need to canonicalize
repeatedly pay the cost of loading the design system each time.

With `--stream`, a tool can start `tailwindcss canonicalize --stream`
once and send candidate groups over stdin as needed:

```sh
$ echo -e "py-3 p-1 px-3\nmt-2 mr-2 mb-2 ml-2" | tailwindcss canonicalize --stream
p-3
m-2
```

Related discussion:
https://github.com/tailwindlabs/tailwindcss/discussions/19736

---------

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