Add skeleton loader for file tree browser
Context
See #582189 (comment 2927656198).
Do we want to have skeleton loader?
Environment: Production Browser: Chrome 142 OS: macOS Viewport: Desktop Screen Reader: [If applicable: VoiceOver, NVDA, JAWS, Orca - include version]
Evidence
it's a small delay with my internet. can definitely get worse
Proposal
Use a skeleton loader to load chunks of the file tree browser when there is a large list.
Edited by 🤖 GitLab Bot 🤖