feat(open): add window_picker.picker

Author: alex-courtis

lua/nvim-tree/actions/node/open-file.lua

@@ -175,7 +175,7 @@ local function get_target_winid(mode, win_ids)
     end
   else
     -- pick a window
-    if M.window_picker.picker then
+    if type(M.window_picker.picker) == "function" then
       target_winid =  M.window_picker.picker()
     else
       target_winid = pick_win_id()