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

spec/lib/discourse_js_processor_spec.rb

@@ -159,7 +159,11 @@ RSpec.describe DiscourseJsProcessor do
 
     let(:mini_racer) do
       ctx = MiniRacer::Context.new
-      ctx.eval(File.open("#{Rails.root}/node_modules/handlebars/dist/handlebars.js").read)
+      ctx.eval(
+        File.open(
+          "#{Rails.root}/app/assets/javascripts/discourse/node_modules/handlebars/dist/handlebars.js",
+        ).read,
+      )
       ctx.eval(helpers)
       ctx
     end