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/discourse_js_processor.rb

@@ -66,7 +66,12 @@ class DiscourseJsProcessor
 
     def self.build_theme_transpiler
       FileUtils.rm_rf("tmp/theme-transpiler") # cleanup old files - remove after Jan 2025
-      Discourse::Utils.execute_command("node", "app/assets/javascripts/theme-transpiler/build.js")
+      Discourse::Utils.execute_command(
+        "pnpm",
+        "-C=app/assets/javascripts/theme-transpiler",
+        "node",
+        "build.js",
+      )
     end
 
     def self.build_production_theme_transpiler