Should extract css from js to improve performance

[benjiwheeler]

We should extract css from js in our webpack asset compilation process. We might use the mini-css-extract-plugin to extract our styles into stylesheets and include them in the template.ejs file.

This would reduce the size of our JS and allow the browser to parallelize the loading of scripts/stylesheets.

There are two open PRs that address this:

• Extract css from js into stylesheets scratch-www#5720 by @paulkaplan in 2021
• Extract css from the JS #4521 by @mzgoddard in 2019

The discussion in #4521 makes it sound like this change would require changes in several other repos; unclear if that is also true of scratchfoundation/scratch-www#5720