feat: add sort_by option to sort files by modification time (#1040)

Co-authored-by: Brandon Dwiel <bdwiel@apple.com>

doc/nvim-tree-lua.txt

@@ -83,6 +83,7 @@ function.
       auto_close           = false,
       auto_reload_on_write = true,
       open_on_tab          = false,
+      sort_by              = "name",
       hijack_cursor        = false,
       update_cwd           = false,
       hijack_unnamed_buffer_when_opening = false,
@@ -201,6 +202,12 @@ Here is a list of the options available in the setup call:
   type: `boolean`
   default: `false`
 
+*nvim-tree.sort_by*
+- |sort_by|: changes how files within the same directory are sorted. can be
+  one of 'name' | 'modification_time'
+  type: `string`
+  default: `"name"`
+
 *nvim-tree.hijack_unnamed_buffer_when_opening*
 - |hijack_unnamed_buffer_when_opening|: opens in place of the unnamed
   buffer if it's empty.