Refactor webpack rule test to support 'postcss' as a lang/file-extension

[enjibby]

Laravel Mix and VSCode's Vetur plugin are quite at odds with how to specify that you wish to use PostCSS in your VueJS single-file component. I have used some custom configuration in my project to support <style lang="postcss"> for my NPM builds, but Storybook (in particular, this postcss addon) does not yet recognise this customisation.

I have updated the css rule test to allow a developer to specify <style lang="postcss"> in their single-file components to ensure Vetur and linting tools correctly recognise the code, while still supporting the same compilation when using the default <style> element.