Normalize and enforce single quotes in styles

- Replace stylelint property in package.json with .stylelintrc.js
- Add string-quotes stylelint rule
- Fix quotes

resources/assets/styles/autoload/_bootstrap.scss

@@ -1 +1 @@
-@import "~bootstrap/scss/bootstrap";
+@import '~bootstrap/scss/bootstrap';

resources/assets/styles/common/_variables.scss

@@ -1,5 +1,5 @@
 /** Import Bootstrap functions */
-@import "~bootstrap/scss/functions";
+@import '~bootstrap/scss/functions';
 
 $theme-colors: (
   primary: #525ddc

resources/assets/styles/components/_comments.scss

@@ -10,14 +10,14 @@
   @extend .form-group;
 }
 
-.comment-form input[type="text"],
-.comment-form input[type="email"],
-.comment-form input[type="url"],
+.comment-form input[type='text'],
+.comment-form input[type='email'],
+.comment-form input[type='url'],
 .comment-form textarea {
   @extend .form-control;
 }
 
-.comment-form input[type="submit"] {
+.comment-form input[type='submit'] {
   @extend .btn;
   @extend .btn-secondary;
 }

resources/assets/styles/main.scss

@@ -1,7 +1,7 @@
-@import "common/variables";
+@import 'common/variables';
 
 /** Import everything from autoload */
-@import "./autoload/**/*";
+@import './autoload/**/*';
 
 /**
  * Import npm dependencies
@@ -9,17 +9,17 @@
  * Prefix your imports with `~` to grab from node_modules/
  * @see https://github.com/webpack-contrib/sass-loader#imports
  */
-// @import "~some-node-module";
+// @import '~some-node-module';
 
 /** Import theme styles */
-@import "common/global";
-@import "components/buttons";
-@import "components/comments";
-@import "components/forms";
-@import "components/wp-classes";
-@import "layouts/header";
-@import "layouts/sidebar";
-@import "layouts/footer";
-@import "layouts/pages";
-@import "layouts/posts";
-@import "layouts/tinymce";
+@import 'common/global';
+@import 'components/buttons';
+@import 'components/comments';
+@import 'components/forms';
+@import 'components/wp-classes';
+@import 'layouts/header';
+@import 'layouts/sidebar';
+@import 'layouts/footer';
+@import 'layouts/pages';
+@import 'layouts/posts';
+@import 'layouts/tinymce';