🐛 BUG: Skypack build fails if using modern JS features

[LeaVerou]

Quick checklist

• I am using the latest version of Snowpack and all plugins.

What package manager are you using?

npm

What operating system are you using?

macOS

Describe the bug

I tried to use Skypack in Codepen to import colorjs.io but the build failed due to the use of private class features.
Note that these have been supported in Node since Node 12 and in browsers since 2020.

Steps to reproduce

See testcase

Link to minimal reproducible example (optional)

https://codepen.io/leaverou/pen/OJvVWay