Add stylelint-scss plugin

This adds extra SCSS-specific rules. In particular, there's a modified at rule
processing rule which understands `mixin` and friends.

.stylelintrc.js

@@ -1,5 +1,8 @@
 module.exports = {
     "extends": "stylelint-config-standard",
+    "plugins": [
+        "stylelint-scss",
+    ],
     "rules": {
         "indentation": 4,
         "comment-empty-line-before": null,
@@ -11,5 +14,7 @@ module.exports = {
         "number-no-trailing-zeros": null,
         "number-leading-zero": null,
         "selector-list-comma-newline-after": null,
+        "at-rule-no-unknown": null,
+        "scss/at-rule-no-unknown": true,
     }
 }

package.json

@@ -153,6 +153,7 @@
     "source-map-loader": "^0.2.3",
     "stylelint": "^9.10.1",
     "stylelint-config-standard": "^18.2.0",
+    "stylelint-scss": "^3.9.0",
     "walk": "^2.3.9",
     "webpack": "^4.20.2",
     "webpack-cli": "^3.1.1"

yarn.lock

@@ -2295,6 +2295,11 @@ crypto-browserify@^3.11.0:
     randombytes "^2.0.0"
     randomfill "^1.0.3"
 
+cssesc@^3.0.0:
+  version "3.0.0"
+  resolved "https://registry.yarnpkg.com/cssesc/-/cssesc-3.0.0.tgz#37741919903b868565e1c09ea747445cd18983ee"
+  integrity sha512-/Tb/JcjK111nNScGob5MNtsntNM1aCNUDipB/TkwZFhyDrrE47SOx/18wF2bbjgc3ZzCSKW1T5nt5EbFoAz/Vg==
+
 currently-unhandled@^0.4.1:
   version "0.4.1"
   resolved "https://registry.yarnpkg.com/currently-unhandled/-/currently-unhandled-0.4.1.tgz#988df33feab191ef799a61369dd76c17adf957ea"
@@ -5977,6 +5982,15 @@ postcss-selector-parser@^3.1.0:
     indexes-of "^1.0.1"
     uniq "^1.0.1"
 
+postcss-selector-parser@^6.0.2:
+  version "6.0.2"
+  resolved "https://registry.yarnpkg.com/postcss-selector-parser/-/postcss-selector-parser-6.0.2.tgz#934cf799d016c83411859e09dcecade01286ec5c"
+  integrity sha512-36P2QR59jDTOAiIkqEprfJDsoNrvwFei3eCqKd1Y0tUsBimsq39BLp7RD+JWny3WgB1zGhJX8XVePwm9k4wdBg==
+  dependencies:
+    cssesc "^3.0.0"
+    indexes-of "^1.0.1"
+    uniq "^1.0.1"
+
 postcss-syntax@^0.36.2:
   version "0.36.2"
   resolved "https://registry.yarnpkg.com/postcss-syntax/-/postcss-syntax-0.36.2.tgz#f08578c7d95834574e5593a82dfbfa8afae3b51c"
@@ -5987,6 +6001,11 @@ postcss-value-parser@^3.3.0, postcss-value-parser@^3.3.1:
   resolved "https://registry.yarnpkg.com/postcss-value-parser/-/postcss-value-parser-3.3.1.tgz#9ff822547e2893213cf1c30efa51ac5fd1ba8281"
   integrity sha512-pISE66AbVkp4fDQ7VHBwRNXzAAKJjw4Vw7nWI/+Q3vuly7SNfgYXvm6i5IgFylHGK5sP/xHAbB7N49OS4gWNyQ==
 
+postcss-value-parser@^4.0.0:
+  version "4.0.0"
+  resolved "https://registry.yarnpkg.com/postcss-value-parser/-/postcss-value-parser-4.0.0.tgz#99a983d365f7b2ad8d0f9b8c3094926eab4b936d"
+  integrity sha512-ESPktioptiSUchCKgggAkzdmkgzKfmp0EU8jXH+5kbIUB+unr0Y4CY9SRMvibuvYUBjNh1ACLbxqYNpdTQOteQ==
+
 postcss@^7.0.0, postcss@^7.0.1, postcss@^7.0.13, postcss@^7.0.14, postcss@^7.0.16, postcss@^7.0.2, postcss@^7.0.5, postcss@^7.0.7:
   version "7.0.17"
   resolved "https://registry.yarnpkg.com/postcss/-/postcss-7.0.17.tgz#4da1bdff5322d4a0acaab4d87f3e782436bad31f"
@@ -7364,6 +7383,17 @@ stylelint-config-standard@^18.2.0:
   dependencies:
     stylelint-config-recommended "^2.2.0"
 
+stylelint-scss@^3.9.0:
+  version "3.9.0"
+  resolved "https://registry.yarnpkg.com/stylelint-scss/-/stylelint-scss-3.9.0.tgz#45e5e42c28c180ec410ac95ab70fb0b49688cbb6"
+  integrity sha512-6uB2TPfIA/HybB7gERPFaBWHDBWhoLSjKoG0rQtsQ4LNOvWzJ205Q8JA2x9zZSXOJXatonlT9aGA1tyNBJrmcg==
+  dependencies:
+    lodash "^4.17.11"
+    postcss-media-query-parser "^0.2.3"
+    postcss-resolve-nested-selector "^0.1.1"
+    postcss-selector-parser "^6.0.2"
+    postcss-value-parser "^4.0.0"
+
 stylelint@^9.10.1:
   version "9.10.1"
   resolved "https://registry.yarnpkg.com/stylelint/-/stylelint-9.10.1.tgz#5f0ee3701461dff1d68284e1386efe8f0677a75d"