Add support for the extracted frontend gem in the generated Gemfile

Since Solidus v3.2 the frontend gem is in its own repo.

Author: elia

lib/solidus_dev_support/templates/extension/Gemfile

@@ -6,6 +6,10 @@ git_source(:github) { |repo| "https://github.com/#{repo}.git" }
 branch = ENV.fetch('SOLIDUS_BRANCH', 'master')
 gem 'solidus', github: 'solidusio/solidus', branch: branch
 
+# The solidus_frontend gem has been pulled out since v3.2
+gem 'solidus_frontend', github: 'solidusio/solidus_frontend' if branch == 'master'
+gem 'solidus_frontend' if branch >= 'v3.2' # rubocop:disable Bundler/DuplicatedGem
+
 # Needed to help Bundler figure out how to resolve dependencies,
 # otherwise it takes forever to resolve them.
 # See https://github.com/bundler/bundler/issues/6677