[Meta] Navigate repo with a file tree - initial tasks for 18.0

Context

This issue is meant to track the efforts done in 18.0 to execute on the outcome of the spike: #525169 (closed)

Tasks

• Terminology:
  • *Existing file tree → File tree
  • *New file tree browser component → File tree browser (FTB)

Task	Status	Notes
Add FF + base components for file tree browser (FTB)	!189445 (merged)
Render a basic FTB + files next to the repository file tree	!190487 (merged)
Render a basic FTB + files next to the blob viewer	!190487 (merged)
Indicate the current file in the FTB	!190487 (merged)
Implement filtering	(!190487 (merged) adds basic filtering), need a follow-up to improve filtering
Navigate from tree → tree via FTB without refresh	!190487 (merged)
Navigate from tree → blob via FTB without refresh	#541099 (closed)
Navigate from blob → tree vis FTB without refresh	#541099 (closed)	Requires passing extra props for tree to render
Navigate from overview → tree/blob without refresh	#541099 (closed)	FTB should not render on overview page
Share state between FTB and tree	#541100 (closed)
Implement file browser panel resizer	#541104 (closed)
Implement expand/collapse toggle + auto-collapse on mobile	#541102 (closed)
Match the styling of MR file browser + responsiveness	#541106 (closed)
Implement ARIA tree view pattern	#541107	See #525169 (comment 2447409353)