fix(types): Missing exports on middleware

[stephenwf]

Missing exports on middleware required for generating declarations (fixes #1103).

As this requires a different tsconfig.json entry for the compiler to complain, I could not think of a simple way to integrate with the existing type-testing.

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zustand-4.0.0-rc.2-declaration	Issue #1103
zustand-4.0.0-rc.2-declaration-from-docs	Issue #1103

[dai-shi]

Thanks for the suggestion.
It turns out that #1012 was a wrong move. #1101 (comment)

#1106 reverts it.