fix source maps for dev server

see rails/sprockets-rails#515

Author: thejonroberts

Procfile.dev

@@ -1,3 +1,3 @@
 web: env RUBY_DEBUG_OPEN=true bin/rails server
-js: yarn build --watch
+js: yarn build:dev --watch
 css: yarn build:css --watch

package.json

@@ -9,6 +9,7 @@
   },
   "scripts": {
     "build": "esbuild app/javascript/*.* --bundle --sourcemap --format=esm --outdir=app/assets/builds --public-path=/assets",
+    "build:dev": "esbuild app/javascript/*.* --bundle --sourcemap --format=esm --outdir=app/assets/builds",
     "build:css": "sass ./app/assets/stylesheets/application.sass.scss:./app/assets/builds/application.css --no-source-map --load-path=node_modules"
   }
 }