Include elemId into TypeScript definition of Op
Include elemId into TypeScript definition of Op

(follow-up on #424)

When generation `automerge.Change`, we need to include `startOp` field to be able to encode it using `automerge.encodeChange`. In order to do this, we need to include `startOp` into TypeScript interface of `Change`.
automerge/automerge,automerge/automerge