Fix postcss-scss module resolution when other postcss versions are present

[tom-ricci]

Due to a bug with dependency resolution, when multiple versions of the postcss package are present, this config will fail to load postcss-scss. Normally this isn't an issue, but there are still cases where other postcss versions are needed (for example create-react-app 4 doesn't support postcss 8).

Based on stylelint/stylelint#5632 (comment), a fix should be made as such in index.js:

- customSyntax: 'postcss-scss',
+ customSyntax: require('postcss-scss'),

Eventually once the postcss community moves on from postcss 7, this should be okay to remove.