CJS support and other `onLoad` returns

[AndrewLeedham]

Hi, I love the idea of this package. esbuild has always been missing piped plugins IMO. I ran into a few issues trying to add support to unplugin and vanilla-extract:

1. The package only ships ESM, so in some environments it is not usable, my specific use case was a vite.config.ts file, as they are transpiled to CJS before running.
2. Errors/warnings/loader and other onLoad returns are lost between each plugin. Perhaps errors/warnings can be concatenated and the loader can be retained from the last plugin or derived from the file name?

Curious if you are open to accepting PRs for some of the above. I know adding new feature can add maintenance overhead, so don't want to spend the time adding things without your blessing 🙂