[TreeView] Add area for `dragHandle` to be shown before the expand/collapse toggle

[ayy-bc]

• add an area in the grid to add the drag handle (before any other icon/button/component)
• drag handle should be visible when hovering over the TreeView.Item
• only take up the space if drag handle is passed from the consumer to tree view
• when drag handle is passed from consumer, shift child level items to match alignment

https://github.com/github/github/pull/321236/files#diff-785247148fc29c46a71b4de392214511ae6709b4516cc4e921d5e9b37b5a23d3

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