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

lib/ember_cli.rb

@@ -41,7 +41,10 @@ class EmberCli < ActiveSupport::CurrentAttributes
   def self.ember_version
     @version ||=
       begin
-        ember_source_package_raw = File.read("#{Rails.root}/node_modules/ember-source/package.json")
+        ember_source_package_raw =
+          File.read(
+            "#{Rails.root}/app/assets/javascripts/discourse/node_modules/ember-source/package.json",
+          )
         JSON.parse(ember_source_package_raw)["version"]
       end
   end