Switch to Turbopack

[kmonahan]

Next.js 16 defaults to Turbopack, but we currently have a custom webpack setup. Need to see if we can migrate that to Turbopack options and then remove the --webpack flags from package.json scripts.

[kmonahan]

Tested upgrading to Turbopack and found some issues that will need to be resolved first.

1. The WP starter uses a banner plugin that doesn't have a Turbopack equivalent
2. Turbopack isn't playing nicely with postcss-import. Layer imports are incorrectly transformed to @media layer() instead of @layer. This doesn't seem to happen with webpack.

Stylelint will also no longer run during the build process but only during the linting script. I think that's fine though, since that's how we're handling linting for JS and TS type checking.

#2 is the larger blocker.

[kmonahan]

Issue is likely related to the fact that Next.js with webpack uses PostCSS but Next.js with Turbopack uses LightningCSS. As of Next.js 15, there is no option to use another compiler with Turbopack.

[kmonahan]

Digging in deeper, I can get around the layer transform issue, but Turbopack/LightningCSS handles @import, which seems to break postcss-advanced-variables, since it cannot find mixins. We may need to reconsider our use of mixins, which would be a larger refactoring of the CSS 🤔