This PR adds a filetree to the left side of the files/diff view.
Initially the filetree will not be shown and may be shown via a new
"Show file tree" button.
Showing and hiding is using the same icon as github. Folders are
collapsible. On small devices (max-width 991 PX) the file tree will be
hidden.
Close #18192
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
