Remove ts-essentials dependency

In https://github.com/mobxjs/mobx-state-tree/pull/2245, we added ts-essentials as a dependency, which is undesirable in the long term. Our goal is to have MST be dependency-free (other than a peer dependency on MobX, and dev dependencies as needed).

We should find a way to get rid of that dependency. For now it's a very small, TS-only dep, and including it fixes bugs out in production.