Basic accessibility and semantics for file browser

What does this MR do and why?

This MR fixes accessibility issues with File browser in merge requests:

1. Uses proper <button> tag for file tree entries
2. Removes <button> tag from the status icon
3. Divides file browser into 2 sections: header and file tree navigation
4. Hides total files counter from screen readers because it's redundant (the same counter is already present on the page)

Screenshots or screen recordings

No visual change.

Screen_Recording_2025-04-30_at_05.37.16

How to set up and validate locally

1. Go to any merge request
2. Select 'Changes' tab
3. Open the file browser
4. Use a screen reader to navigate through files in the file browser
5. Basic features should work, regions should be properly announced