Autosplitter for Livesplit. It will automatically be downloaded if you set your game to Manifold Garden. It has been tested on version 14704 (the "Speedrunning branch" on Steam) and 15769 (Steam latest), but should also work on other versions.

If you want to customise which levels it splits at, you can create a file with a .mg_config extension, and put it in the same folder as your splits. This file should contain a list of level names, one per line, and the names must exactly match the in-game names shown by the toggle_beta_message feature. If you set that up correctly, then the config file should show up in the autosplitter settings.