api.tree.open target_window

[alex-courtis]

Background #2173

Aerial added aerial.open_in_win stevearc/aerial.nvim#248 allowing the user to specify a target window instead of opening a new one via the usual rules.

Add similar functionality to nvim-tree.

Allows new workflow e.g.

• create windows for aerial and nvim-tree
• open aerial in one
• open nvim-tree in the other

This is necessary as nvim does not allow window repositioning outside of rows/columns.