chore: move preprocessor deps to peerDependencies

Author: ferferga

README.md

@@ -39,6 +39,11 @@ See [documents](https://future-architect.github.io/eslint-plugin-vue-scoped-css/
 npm install --save-dev eslint eslint-plugin-vue-scoped-css vue-eslint-parser
 ```
 
+If you use SCSS or STYLUS in your project, you also need to run:
+
+* SCSS: `npm install --save-dev postcss-scss`
+* Stylus: `npm install --save-dev postcss-styl`
+
 > **Requirements**
 >
 > - ESLint v6.0.0 and above

package-lock.json

@@ -108,12 +108,12 @@
     "lodash": "^4.17.21",
     "postcss": "^8.4.31",
     "postcss-safe-parser": "^6.0.0",
-    "postcss-scss": "^4.0.3",
-    "postcss-selector-parser": "^7.0.0",
-    "postcss-styl": "^0.12.0"
+    "postcss-selector-parser": "^7.0.0"
   },
   "peerDependencies": {
     "eslint": ">=5.0.0",
+    "postcss-scss": "^4.0.3",
+    "postcss-styl": "^0.12.0",
     "vue-eslint-parser": ">=7.1.0"
   },
   "nyc": {