Resolve "Decouple multi-file editor from file list"

What does this MR do?

Integrates the Multi File Editor as an Beta Preview feature to start exploring its capabilities and as a foundation for all upcoming tasks. For now this feature will be hidden behind the existing cookie flag.

• Adds the multi file editor as a stand alone feature on its own route (/-/ide)
• Everything after /-/ide/* will also be caught as a wildcard and then handed over to the Vue application
• Adds vue-router for the routing in the Vue application
• Changes the vuex data structure to be multi project and multi branch capable
• Adds basic IDE design (see screenshots)
• Collapsable sidebars
• Status Bar with info about the current file

All additional features/imrpovements will be handled in the upcoming MR's.

EE Port : https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3846

Are there points in the code the reviewer needs to double check?

Backend:

• New route
• Added URL to a new MR to the API

Why was this MR needed?

To start the IDE Beta!

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #40040 (closed), #40032 (closed)