Resolve "Helpful and instructing Empty state for multi file editor"

What does this MR do?

• Adds a nice empty state illustration + better text
• Skeleton Loader for the left sidebar
• Top Menu Web IDE button to represent where you are (for now its only visible if you are on that route)

Why was this MR needed?

To make the Web IDE prettier :-)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests edited for this feature/bug
• Review
  • Has been reviewed by UX
  • 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?

Main Menu Button from #41161 (moved)

Closes #41048 (closed), #41641 (closed)