support cjs and esm version for browser builds

[mrtnbroder]

I've been running into issues with basic usage of mobx, where in a complete esm environment, a import * as mobx from 'mobx' webpack would still require the cjs version of mobx, though, a module entry is there and also defined within the package.json. This caused a lot of headache, as there was no reason to do so.

However, due to some weird behaviour, webpack 4 refuses to load the module version of mobx, until I've added the browser field to package.json.

Everything should still work correctly within a commonjs environment, as a require call to mobx within the browser still requires the cjs version. Thanks a lot to https://github.com/stereobooster/main-module-browser-test for the idea behind this.

This works reliable for me, so it would be great to get this merged in!

[coveralls]

Coverage remained the same at 93.577% when pulling 72bdf6b on mrtnbroder:webpack-4-browser-field into d55c50a on mobxjs:master.

[mweststrate]

Wow, ok, thanks for figuring out :)

[mweststrate]

released in 4.8.0 / 5.8.0