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/decorator-transforms@2.0.0.patch

@@ -0,0 +1,21 @@
+diff --git a/dist/index.js b/dist/index.js
+index fce3aeb..c23d8e4 100644
+--- a/dist/index.js
++++ b/dist/index.js
+@@ -4,10 +4,13 @@ import {
+ import "./chunk-CSAU5B4Q.js";
+ 
+ // src/index.ts
+-import { createRequire } from "module";
++
+ import { ImportUtil } from "babel-import-util";
+-var req = createRequire(import.meta.url);
+-var { default: decoratorSyntax } = req("@babel/plugin-syntax-decorators");
++
++// https://github.com/ef4/decorator-transforms/pull/27
++import PluginSyntaxDecorators from "@babel/plugin-syntax-decorators";
++const decoratorSyntax = PluginSyntaxDecorators.default || PluginSyntaxDecorators;
++
+ function makeVisitor(babel) {
+   const t = babel.types;
+   return {