- postcss-loader 4.0 improves compatibility with webpack 5.
- the 4.0.2 and 4.0.3 release notes also mention improved PostCSS 8 compatibility (see issue #17242)
- postcss-loader by default now detects external PostCss configs and merges them with loader options,
  so I have disabled this to preserve Next’s current behaviour. For example, Next accepts JSON configs with comments,
  but postcss-loader fails to load them.
- when patching the loader configuration, I try to detect the `postcssOptions` field instead of `ident`
  as now passing an `ident` field to the postcss-loader options triggers a schema validation error
- in one error overlay integration test, I check whether the error message contains the expected strings
  instead of matching a snapshot exactly, as now the postcss loader inserts the file path from the filesystem
  root in the error message, which we can’t match exactly across test runs
- updating postcss-loader to 4.0.4 breaks, because `4.0.4` requires schema-utils `^3.0.0`, but the ncc build
  prevents postcss-loader from using its own schema-utils