Update USWDS package to 3.0

[fwextensions]

Sass has deprecated @import in favor of @use, which v3 of the package is using.

https://designsystem.digital.gov/documentation/migration/
https://designsystem.digital.gov/documentation/settings/

[fwextensions]

This branch tries to address this, but it doesn't fully work. npm run sass compiles okay, but the sass command in Procfile.dev throws an error about not being able to find the file to import. The same thing happens with webpack in docker.

It may be possible to fix this by changing how the h1 - h4 mixins get overwritten in _uswds-theme-custom-mixins.scss, so that @forward and @use can be used. But lots of component styles include those mixins, and it's not clear what the right way of handling it is.

All in all, trying to update to 3.0 seems like a real sass-ache.