Log to stderr instead of stdout

[jmooring]

Fixes #2203, but needs some more eyeballs on it.

[adamwathan]

Seems good to me, thanks!

[fubhy]

This change causes a regression in snowpack now where output into stderr is considered a failure of the build plugin. Thus, snowpack renders a big fat error message instead of the app whenever we are trying to use experimental features.

https://github.com/pikapkg/snowpack/blob/master/plugins/plugin-build-script/plugin.js#L33-L35

Now in my opinion both tailwind and snowpack are equally "responsible" for this. I don't think it's right to write to "info" level notifications like this to stderr, on the other hand, snowpack shouldn't assume stderr output is a critical failure. They should really just look at the exit code of the underlying script for that. I am going to open a PR in the snowpack repository but just wanted to let y'all now and document this problem in case someone else runs into it.

Note: This problem arose for us when we were trying to use the new dark mode experimental feature in snowpack through a snowpack build script:

Build Error: build:postcss
Error: 
warn - You have enabled experimental features: darkModeVariant
warn - Experimental features are not covered by semver, may introduce breaking changes, and can change at any time.

./cc @adamwathan in case you want to re-consider this change also for tailwind?

[adamwathan]

@fubhy Thanks for reporting, can you please open a new issue? I will 100% forget about this since there's no way for us to track comments on closed PRs as active things we need to investigate.