Exports a couple of project related code as es6 modules

What does this MR do?

Exports as es6 modules:

1. app/assets/javascripts/project_label_subscription.js
2. app/assets/javascripts/project_new.js
3. app/assets/javascripts/project_select.js
4. app/assets/javascripts/project_variables.js

Removes app/assets/javascripts/project_show.js

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

#38869 (moved)