Move asset compilation config to recommended location

[orien]

Context

The Rails Engine Guide recommends to put asset compilation configuration in the engine.rb file:

initializer "blorgh.assets.precompile" do |app|
  app.config.assets.precompile += %w( admin.js admin.css )
end

https://guides.rubyonrails.org/v6.1/engines.html#separate-assets-and-precompiling

Change

• Move the asset compilation configuration.
• Prepare the release of 0.7.0

[petervandoros]

LGTM 👍 assuming you've tested the changes.

[orien]

Everything looks good