Remove merge request code from window object and enable eslint

What does this MR do?

Export MergeRequest and MergeRequestTabs as ES6 Modules Enables eslint

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

What are the relevant issue numbers?

#38869 (moved)