Improve Typescript types and generics

[Lynncubus]

Fixes #21

Exports a few types for usage outside of the module and adds default types for generics.

[drcmda]

Looks good to me, we need these types as well at work, duplicated them for now. @JeremyRH how do you feel? You still wanted to remove the internal types, like StateSelector, is this correct?

[JeremyRH]

The reason for not wanting to expose all the types is mostly for type names. If we change a name, it's a breaking change. I do see the value for not duplicating types though so I'm thinking we should expose all the types.

I'll test out these changes today and try to get this merged. Thanks @Luxizzle!

[dm385]

When using "zustand" in one of our projects, I had defined the types UseStore and StoreApi in globals.d.ts myself. Now I came across this pull request. If I now compile the project with Luxizzle's solution directly with tsc, I get some compiler errors. These can also be reproduced in this repo when compiling the file test.tsx. For this reason, I have adapted the definitions in my own branch and adapted the configuration so that it can be compiled directly with tsc (yarn build:tsc).

You can have a look at it if you like:
improve-typescript

Please correct me if I am wrong.

[JeremyRH]

@dm385 You're right, the tests were failing with Luxizzle's changes. I asked them to create a PR so we could work on fixing the issues together. I have a version of their changes working without changing the tests now.

And thank you for the fixes on your fork. I'll see if there's anything there that I can add to this.

[Lynncubus]

I updated everything with the proposed changes you gave. Thanks for all the explanations by the way, they really helped!

[JeremyRH]

Thanks!