Bug: Microbundle generates 'x.modern.css' and 'x.module.css' files

[rschristian]

As the title indicates, as of v13, Microbundle generates x.modern.css and x.module.css (and their source maps). The UMD build, however, seems to be safe from this as there is not a x.umd.css.

Here's a repo that one can use to reproduce: https://github.com/rschristian/microbundle-css-output

I've pushed a build to the repo for easier viewing.