Ensure TreeView cache entry is removed on delete

Address issue:

https://github.com/lite-xl/lite-xl/issues/689

Attempt to provide a more accurate fix to commit:

59f64088e1

For this latter what happens is that any change inside a directory
cause the corresponding entry to be folded in the TreeView.

The new change is more accurate because we remove only the stale
entry corresponding to the delete event and we do not reset the
cache of the parent directory using the modify event.

data/core/init.lua

@@ -1164,8 +1164,8 @@ end
 
 
 -- no-op but can be overrided by plugins
-function core.on_dirmonitor_modify()
-end
+function core.on_dirmonitor_modify() end
+function core.on_dirmonitor_delete() end
 
 
 function core.on_dir_change(watch_id, action, filepath)
@@ -1174,6 +1174,7 @@ function core.on_dir_change(watch_id, action, filepath)
   core.dir_rescan_add_job(dir, filepath)
   if action == "delete" then
     project_scan_remove_file(dir, filepath)
+    core.on_dirmonitor_delete(dir, filepath)
   elseif action == "create" then
     project_scan_add_file(dir, filepath)
     core.on_dirmonitor_modify(dir, filepath);