Add precompile hook

[justin808]

Sometimes, we always want to run some code, like

rake react_on_rails:generate_packs

whenever about to run webpack

We can add a shakapacker.yml configuration so some code can run before invoking the webpack.

precompile_hook: rake react_on_rails:generate_packs

This way, "it just works" in terms of having some files ready that React on Rails needs.

Alternatives:

• While we could modify the installed bin files, that can make upgrades trickier
• We could wrap the bin files with new scripts.

[G-Rath]

You should already be able to do this with Rake:

# lib/tasks/assets.rake
namespace :assets do
  desc "Ensures that dependencies required to compile assets are installed"
  task install_dependencies: :environment do
    raise if File.exist?("package-lock.json") && !(system "npm install")
  end
end

Rake::Task["assets:precompile"].enhance ["assets:install_dependencies"]