Replace gulp-sass-lint with gulp-postcss / stylelint

[agibson-godaddy]

⚠️ Merge #91 first

Issue: https://godaddy-corp.atlassian.net/browse/MWC-17960

Summary

This replaces the unmaintained gulp-sass-lint with gulp-postcss / stylelint. It also adds a lint config, which disables a ton of rules just to avoid suddenly switching to this method and ending up with a ton of warnings you didn't have before. IMO it's up to individual plugins to override these lint rules (supported).

To override:

Create a new .stylelintrc.json file in plugin directory with desired rules.

Update sake config:

  // sake.config.js
  module.exports = {
    options: {
      'stylelint-configFile': '.stylelintrc.json'  // Path relative to project root
    }
    // ... other sake configuration
  }