Explicitly add postcss to dependencies

stylelint-config-gds includes postcss-scss, this expects postcss as peer
depdendency, by default, other dependencies use the wrong version. Needed
to install as dependency so that the correct version is available. More
info here:
stylelint-scss/stylelint-config-standard-scss#5

package.json

@@ -11,7 +11,8 @@
     "@rails/ujs": "^7.0.2",
     "@rails/webpacker": "5.4.3",
     "govuk-frontend": "^4.0.1",
-    "jquery": "3.6.0"
+    "jquery": "3.6.0",
+    "postcss": "^8.4.12"
   },
   "devDependencies": {
     "standard": "^16.0.4",