Add stylelint-scss rules

[rynpsc]

This PR disables rules from the stylelint-scss plugin that are unnecessary or might conflict with Prettier, closes #105. These rules are included within in main config rather than a seperate config and matches eslint-config-prettier. If the user does not have any scss rules configured they are ignored, therefore we don't need to introduce stylelint-scss as a peer dependency.

To enable support I've had to modify the checker as if a stylelint-scss rule is set to null in the users config i.e disabled, stylelint-scss reports the setting as an empty array rather than null. Therefore I've updated the script to check for both null and an empty array.

[shannonmoeller]

Thank you for the PR. Looks good.

I don't use Sass much anymore and I'd like to create more robust tests to prove how configs prevent conflicts eventually, but until then I'm just going to ask: How confident are you that these changes are necessary, correct, and complete?

[rynpsc]

Yes more tests would definitely be beneficial. For now my testing has been largely manual using the Prettier REPL and testing the examples within the stylelint-scss docs for all the rules that relate to style to see what Prettier would do to them. I then added add rules where Prettier would format them differently regardless of input. Most of the included rules are around if and else statements where no matter what you pass it Prettier does its own thing.

[EvgenyOrekhov]

👍

[shannonmoeller]

Released as v9.