refacto: create actions folder and move some code to it

- move trash.lua into actions
- move system-open from root file to actions
- move copypasta code from fs to actions
- add file_exists in utils
- add rename_loaded_buffers in utils (might need to move that in the
  future).

lua/nvim-tree/actions/init.lua

@@ -0,0 +1,8 @@
+local M = {}
+
+function M.setup(opts)
+  require'nvim-tree.actions.system-open'.setup(opts.system_open)
+  require'nvim-tree.actions.trash'.setup(opts.trash)
+end
+
+return M