Unify bin/dev script with other js/css installers

[marcoroth]

This Pull Request streamlines the content of the bin/dev script across rails/dartsass-rails, rails/jsbundling-rails and rails/cssbundling-rails so you don't run into conflicts when running installer:

❯ rails dartsass:install
Build into app/assets/builds
       exist  app/assets/builds
   identical  app/assets/builds/.keep

Stop linking stylesheets automatically
        gsub  app/assets/config/manifest.js

Add default app/assets/stylesheets/application.scss
      create  app/assets/stylesheets/application.scss
      append  Procfile.dev

Add bin/dev to start foreman
    conflict  bin/dev
Overwrite /Users/marcoroth/Development/project/bin/dev? (enter "h" for help) [Ynaqdhm] 

This PR applies the changes from rails/cssbundling-rails#93 and rails/cssbundling-rails#98