Add `effectOn` API

So I was trying to figure out a way to centralize an effect based on when a specific part of a model changes. This should cover when it loads from default to persisted value. The idea would be something like:

const storeModel = persist({
  token: '',
  setToken: action((state, payload) => ({ ...state, token: payload })),
  onTokenChange: effectOn(state => state.token, async () => {
    // fetch user data and dispatch actions.
  })
})

This would allow us to fetch user data when token is changed from anywhere, whether it be from an action, or from the initial load from the persisted store.

The specific api isn’t important, but I think this mechanism could allow much easier centralization of logic.

What do ya’ll think?

[ctrlplusb]

Great! Yep, plugin architecture is still on the way. I did a trial run on another branch, but will apply it to 3.4.0 soon. Will ping you when it is ready.

[ctrlplusb]

@paul-sachs the code, tests and TS typings are complete. 👍

I just need to write docs then will release into alpha.