feat(explorer): add filesystem watchers (#1304)

* feat(explorer): add experimental watchers

This commit introduces watchers to update the tree.
This behavior is introduced behind an "filesystem_watchers" option
which should prevent instabilities.
It will become the default at some point.

Co-authored-by: Alexander Courtis <alex@courtis.org>

lua/nvim-tree/actions/system-open.lua

@@ -51,7 +51,7 @@ function M.fn(node)
 end
 
 function M.setup(opts)
-  M.config.system_open = opts or {}
+  M.config.system_open = opts.system_open or {}
 
   if #M.config.system_open.cmd == 0 then
     if M.config.is_windows then