Add PNPM support (#287)

Author: pierry01

lib/generators/ruby_ui/javascript_utils.rb

@@ -8,6 +8,8 @@ def install_js_package(package)
           run "yarn add #{package}"
         elsif using_npm?
           run "npm install #{package}"
+        elsif using_pnpm?
+          run "pnpm install #{package}"
         else
           say "Could not detect the package manager, you need to install '#{package}' manually", :red
         end
@@ -30,6 +32,8 @@ def using_importmap?
 
       def using_npm? = File.exist?(Rails.root.join("package-lock.json"))
 
+      def using_pnpm? = File.exist?(Rails.root.join("pnpm-lock.yaml"))
+
       def using_yarn? = File.exist?(Rails.root.join("yarn.lock"))
 
       def pin_motion