feat(types): better middleware support

[dai-shi]

Okay, to be honest, I don't like the tricky typescript typing. middleware is very type unfriendly.
I wouldn't build the API with middleware in this typescript era. (see jotai and valtio.)
That said, we still want to have better TS support in zustand.
Even though this makes the code less maintainable, it's worth it.

(We should add more type tests and middleware behavior tests. Any volunteers?)

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Sandbox	Source
React	Configuration
React Typescript	Configuration
React Browserify	Configuration
React Snowpack	Configuration
React Parcel	Configuration
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[github-actions]

Size Change: 0 B

Total Size: 9.88 kB

ℹ️ View Unchanged

Filename	Size
dist/context.js	622 B
dist/esm/context.js	521 B
dist/esm/index.js	1.16 kB
dist/esm/middleware.js	2.21 kB
dist/esm/shallow.js	272 B
dist/esm/vanilla.js	473 B
dist/index.js	1.29 kB
dist/middleware.js	2.43 kB
dist/shallow.js	318 B
dist/vanilla.js	582 B

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[AnatoleLucet]

If I'm not wrong, these four fields are already in CustomStoreApi, right?
Why not just remove these four lines?

[dai-shi]

Nice catch. At a certain commit, it was interface which can't be merged with CustomStoreApi. Now, it's type alias, and we can remove them.