Add skeleton loader for file tree browser
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Collaborate/take over this issue](https://contributors.gitlab.com/manage-issue?action=work&projectId=278964&issueIid=584240)
</details>
<!--IssueSummary end-->
# Context
See https://gitlab.com/gitlab-org/gitlab/-/issues/582189#note_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
>
> {width=205 height=600}
# Proposal
Use a skeleton loader to load chunks of the file tree browser when there is a large list.
issue