refacto: add get_nodes_starting_line core util

lua/nvim-tree/core.lua

@@ -1,11 +1,11 @@
 local events = require "nvim-tree.events"
 local explorer = require "nvim-tree.explorer"
+local view = require "nvim-tree.view"
 
 local M = {}
 
-local first_init_done = false
-
 TreeExplorer = nil
+local first_init_done = false
 
 function M.init(foldername)
   TreeExplorer = explorer.Explorer.new(foldername)
@@ -23,4 +23,12 @@ function M.get_cwd()
   return TreeExplorer.cwd
 end
 
+function M.get_nodes_starting_line()
+  local offset = 1
+  if view.is_root_folder_visible(M.get_cwd()) then
+    offset = offset + 1
+  end
+  return offset
+end
+
 return M