Namespace helper methods for css:build command

[excid3]

Methods defined in Rake files are added to the global namespace. This means cssbundling-rails and jsbundling-rails using the same build_command method name get overridden by one or the other.

This introduces a module to namespace these methods and provides a place for any future methods to be defined.

Since these only apply to the tasks in build.rake, I just defined the module there, but we could extract it to a file and require it instead.

Fixes #131

cc @terracatta

[excid3]

@rafaelfranca heads up on this and rails/jsbundling-rails#172 👍

[terracatta]

Thanks @excid3 TIL that methods defined here are in a shared namespace with all other rake tasks. I've updated my open PRs as well. hotwired/turbo-rails#494 and hotwired/stimulus-rails#125.