Rename helper to reflect its new role

It handles more than the `defer` attribute now so it's
renamed to GitlabScriptTagHelper

app/helpers/defer_script_tag_helper.rb → app/helpers/gitlab_script_tag_helper.rb

 # frozen_string_literal: true
 
-module DeferScriptTagHelper
+module GitlabScriptTagHelper
   # Override the default ActionView `javascript_include_tag` helper to support page specific deferred loading.
   # PLEASE NOTE: `defer` is also critical so that we don't run JavaScript entrypoints before the DOM is ready.
   # Please see https://gitlab.com/groups/gitlab-org/-/epics/4538#note_432159769.

spec/helpers/defer_script_tag_helper_spec.rb → spec/helpers/gitlab_script_tag_helper_spec.rb

@@ -2,9 +2,9 @@
 
 require 'spec_helper'
 
-RSpec.describe DeferScriptTagHelper do
+RSpec.describe GitlabScriptTagHelper do
   before do
-    allow_any_instance_of(DeferScriptTagHelper).to receive(:content_security_policy_nonce).and_return('noncevalue')
+    allow_any_instance_of(GitlabScriptTagHelper).to receive(:content_security_policy_nonce).and_return('noncevalue')
   end
 
   describe 'external script tag' do