Save yarn locally to workaround setup-node issue

Setup-node refuses to add an option for enabling corepack due to it being marked experimental.
This unwittingly resulted in us using the globally installed yarn v1 version for building, despite our expectation that yarn v4 would be used.
Rather than do a dance of enabling corepack and setting up node, this saves yarn locally and checks it into git.

.yarnrc.yml

@@ -1 +1,3 @@
 nodeLinker: node-modules
+
+yarnPath: .yarn/releases/yarn-4.0.2.cjs

package.json

@@ -54,5 +54,5 @@
     "node": ">=18.0"
   },
   "description": "Website for r/JapanFinance wiki",
-  "packageManager": "[email protected].1+sha256.5a9d054e67fbbae8774cd3667736fd74407ffbbb336898523ca1fe87b3fc4ee3"
+  "packageManager": "[email protected].2"
 }