Update dependency postcss-nesting to v13

Author: renovate[bot]

package.json

@@ -62,7 +62,7 @@
     "postcss-custom-properties": "^14.0.0",
     "postcss-import": "^16.0.0",
     "postcss-mixins": "^11.0.0",
-    "postcss-nesting": "^7.0.1",
+    "postcss-nesting": "^13.0.0",
     "prettier": "^1.19.1",
     "stylelint": "^15.10.1",
     "stylelint-config-prettier": "^8.0.2",