Allow Redux to be imported as type="module"

[Wildhoney]

We'd all love to import Redux as type="module" natively in the browser, however the process.env and the symbol-observable ponyfill make that problematic.

By checking for the existence of symbol-observable ourselves — rather than delegating it to a module unless we really need to — as well as checking for typeof process we can make it happen.

Redux is a simple enough library. Let's all be standards compliant, friends (or as close as we can at this point)! 🍻

Discuss...

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[timdorr]

There is already a mjs build provided by #3143

[Wildhoney]

Thanks @timdorr

I saw the *.mjs file but discounted it without testing it, as it's typically a Node.js convention. I've not seen *.mjs being used as a browser module before anywhere else.

Do you think we need to document its existence? As the current installation section in the README doesn't appear to make reference to it.

Could we also not unify the module files (es/redux.js and es/redux.mjs), which was my attempt with this PR? That way we could reference a single file in the package.json file. At the moment the *.mjs has no reference in the package.json – that coupled with no README mention makes it terribly difficult to locate.

[timdorr]

The mjs file is built in production mode only, so it cannot replace the standard module, which supports both.

Some documentation on it's existence would be good. There isn't yet a package.json field for it that we can use safely, so an entry in the docs would be a good start.