There is no complete list, since user code can pass its own origin string when calling, for example, replaceRange. Some important built-in strings are "+input" (typing), "paste", and "cut". The + character indicates that this kind of event can be merged with other undo history events of the same type.
Thanks for the reply. I actually wasn’t referring to getting them programmatically, I was more asking about those that come with the standard CodeMirror package. Do you know where I can find a list of the ones that CodeMirror itself creates?