Use npm instead of yarn

More in line with classic package manager. Yarn 2+ is just weird.

lib/tasks/jsbundling_build.rake

@@ -0,0 +1,14 @@
+# Mostly lifted from [1] but replace yarn with npm
+# [1] https://github.com/rails/jsbundling-rails/blob/98780f28809ebad5d84921ce38dc238472a35812/lib/tasks/jsbundling/build.rake
+
+# First clear existing task actions
+Rake::Task["javascript:build"].clear_actions
+
+namespace :javascript do
+  desc "Build your JavaScript bundle"
+  task :build do
+    unless system "npm install && npm run build"
+      raise "jsbundling-rails: Command build failed, ensure npm is installed and `npm run build` runs without errors"
+    end
+  end
+end

package.json

@@ -15,7 +15,6 @@
     "mousetrap": "^1.6.5",
     "timeago": "^1.6.7"
   },
-  "devDependencies": {},
   "scripts": {
     "build": "./build.js"
   },