Upgrade yarn to 3.6.4

Version 1 of Yarn is no longer reliably supported, and is only receiving occasional hot fixes on a ‘best efforts’ basis.

Version 3 has been recommended as it has better backwards compatability.
See Yarn's migration steps (linked in pull request).

trello card: https://trello.com/c/AMoqVW5I

.gitignore

@@ -19,3 +19,11 @@
 
 /node_modules
 /yarn-error.log
+
+# https://yarnpkg.com/getting-started/qa#which-files-should-be-gitignored
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions

.yarnrc.yml

@@ -0,0 +1,13 @@
+yarnPath: .yarn/releases/yarn-3.6.4.cjs
+nodeLinker: node-modules
+
+packageExtensions:
+  stylelint-config-recommended-scss@*:
+    peerDependencies:
+      postcss: '8'
+  stylelint-config-standard-scss@*:
+    peerDependencies:
+      postcss: '8'
+  stylelint-config-gds@*:
+    peerDependencies:
+      postcss: '8'

package.json

@@ -15,5 +15,6 @@
     "postcss": "^8.4.31",
     "stylelint": "^14.16.1",
     "stylelint-config-gds": "^0.3.0"
-  }
+  },
+  "packageManager": "[email protected]"
 }