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/core.lua

@@ -9,6 +9,9 @@ TreeExplorer = nil
 local first_init_done = false
 
 function M.init(foldername)
+  if TreeExplorer then
+    TreeExplorer:_clear_watchers()
+  end
   TreeExplorer = explorer.Explorer.new(foldername)
   if not first_init_done then
     events._dispatch_ready()