Make spree_frontend gem optional

[damianlegawiec]

As we're seeing more and more headless installations we're thinking about making the spree_frontend gem fully optional, so it won't install by default with mainspree gem.

This isn't particularly a breaking change because it will mean that developers would need to slightly change their Gemfile from:

gem 'spree'

to

gem 'spree'
gem 'spree_frontend'

We're also thinking about optional storefronts written in other languages than Ruby, mainly JavaScript but that's probably further in the future.

[damianlegawiec]

Done via #11105