change react-dom from devDependency to dependency

[legnaleurc]

Since #1209, we import react-dom explicitly, so maybe it should be put into dependencies instead of devDependencies.
Put it in devDependencies will confuse some plugins of build systems (e.g. Webpack), makes them think react-dom is not needed for react-redux.

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[markerikson]

Appreciate the PR, but no, this is not correct.

Because of how the renderers export unstable_batchedUpdates, we're in a weird situation where we have to import from react-dom, but allow the RN loader to fall back to the RN package instead when used with RN.

Adding react-dom as an explicit dependency would break that, and also force an extra download for all RN users.

In addition, we do need react-dom as a dependency for developing React-Redux.

[legnaleurc]

We need react-dom for deploying react-redux, at least it appears when we run npm run build:umd.
Would it be acceptable to be peerDependencies or optionalDependencies?

[markerikson]

Also no. We don't want to be RN users to be getting warned when they install React-Redux but not ReactDOM. And, since we build the UMDs ourselves as part of the dev process, that's already handled under devDependencies.

I know it's quirky, but what we have now works.

[zkochan]

It can be an optional peer dependency. In that case, users will not be warned about it. Optional peer dependencies are supported by yarn, pnpm and will be supported in npm in the next minor version (thanks to @arcanis)