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

patches/licensee@10.0.0.patch

@@ -0,0 +1,13 @@
+diff --git a/index.js b/index.js
+index 0a0a91720146b84a4b61bd9ec4b573652c133940..064c93ca41b1bb3ffdd9e36a74105146fffb2a9c 100644
+--- a/index.js
++++ b/index.js
+@@ -38,7 +38,7 @@ function licensee (configuration, path, callback) {
+       .then(function (tree) {
+         var dependencies = Array.from(tree.inventory.values())
+           .filter(function (dependency) {
+-            return !dependency.isProjectRoot
++            return !dependency.isProjectRoot && dependency.package.name
+           })
+         if (configuration.filterPackages) {
+           dependencies = configuration.filterPackages(dependencies)