fix(purgecss-webpack-plugin): export as named export as well as default

[G-Rath]

Proposed changes

This should provide a workaround for #820 by enabling importing the class via it's name as well as by default.

Types of changes

What types of changes does your code introduce?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

While the types are still wrong, I think fixing them might be a heavy task as it depends on what is actually building the types & doing the packages - I'm happy to try help with that, but that requires having the whole project setup locally; this workaround will unblock people in the meantime.