chore: refacto setup part 1 (#603)

* chore: refacto setup part 1

refacto setup for code entrypoint
following options switched boolean values as options to the setup function:
- `nvim_tree_disable_netrw` -> `disable_netrw`
- `nvim_tree_hijack_netrw` -> `hijack_netrw`
- `nvim_tree_auto_open` -> `open_on_setup`
- `nvim_tree_auto_close` -> `auto_close`
- `nvim_tree_tab_open` -> `tab_open`
- `nvim-tree-update-cwd` -> `update_cwd`
- `nvim_tree_hijack_cursor` -> `hijack_cursor`
- `nvim_tree_system_open_command` -> `system_open.cmd`
- `nvim_tree_system_open_command_args` -> `system_open.args`
- `nvim_tree_follow` -> `update_focused_file.enable`
- `nvim_tree_follow_update_path` -> `update_focused_file.update_cwd`
Also added new option `update_focused_file.ignore_list` which will
ignore filepath or filetypes that matches one entry of the list when
updating the path if update_cwd is true.

* add deprecation warning

* update readme

* schedule on enter to avoid running before vim first buffer has loaded

* update docs

* correct typo

* rename tab open -> open on tab

lua/nvim-tree/config.lua

@@ -58,8 +58,9 @@ function M.get_icon_state()
     end
   end
 
+  local has_devicons = pcall(require, 'nvim-web-devicons')
   return {
-    show_file_icon = show_icons.files == 1 and vim.g.nvim_web_devicons == 1,
+    show_file_icon = show_icons.files == 1 and has_devicons,
     show_folder_icon = show_icons.folders == 1,
     show_git_icon = show_icons.git == 1,
     show_folder_arrows = show_icons.folder_arrows == 1,