DEV: Switch to pnpm for JS dependencies (#28671)

This will bring significant improvements to install speed & storage requirements. For information on how it may affect you, see https://meta.discourse.org/t/324521

This commit:
- removes the `yarn.lock` and replaces with `pnpm-lock.yaml`
- updates workspaces to pnpm format
- adjusts package dependencies to work with pnpm's stricter resolution strategy
- updates Rails app to load modules from more specific node_modules directories
- adds a `.pnpmfile` which automatically cleans up old yarn-managed `node_modules` directories
- updates various scripts to call `pnpm` instead of `yarn`
- updates patches to use pnpm's native patch system instead of patch-package
- adds a patch for licensee to support pnpm

.pnpmfile.cjs

@@ -0,0 +1,16 @@
+const fs = require("fs");
+const { execSync } = require("child_process");
+
+if (fs.existsSync("node_modules/.yarn-integrity")) {
+  console.log(
+    "Detected yarn-managed node_modules. Performing one-time cleanup..."
+  );
+
+  // Delete entire contents of all node_modules directories
+  // But keep the directories themselves, in case they are volume mounts (e.g. in devcontainer)
+  execSync(
+    "find ./node_modules ./app/assets/javascripts/*/node_modules -mindepth 1 -maxdepth 1 -exec rm -rf {} +"
+  );
+
+  console.log("cleanup done");
+}