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

app/assets/javascripts/custom-proxy/index.js

@@ -244,7 +244,7 @@ module.exports = {
 Discourse can't be run without a \`--proxy\` setting, because it needs a Rails application
 to serve API requests. For example:
 
-  yarn run ember serve --proxy "http://localhost:3000"\n`);
+  pnpm ember serve --proxy "http://localhost:3000"\n`);
       throw "--proxy argument is required";
     }
 

app/assets/javascripts/custom-proxy/package.json

@@ -26,7 +26,7 @@
   },
   "engines": {
     "node": ">= 18",
-    "npm": "please-use-yarn",
-    "yarn": ">= 1.21.1"
-  }
+    "npm": "please-use-pnpm",
+    "yarn": "please-use-pnpm",
+    "pnpm": ">= 9"  }
 }