feat(core): provide previous state to onStateChange

[eunjae-lee]

Summary

This PR adds a new prop prevState to onStateChange.
With this addition, users don't have to keep track of the previous state for comparison.

For example,

      onStateChange({ state }) {
        if (state.query !== queryRef.current) {
          queryRef.current = state.query;
          props.refine(state.query);
        }
      },

becomes

      onStateChange({ state, prevState }) {
        if (state.query !== prevState.query) {
          props.refine(state.query);
        }
      },

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[Haroenv]

only a shallow copy, but since there's no other mutations, that should be all right