Adding rollup to build process

[arthurdenner]

Hi, @mweststrate.

So, I took a deeper look at microbundle and it seems to not be generating sourcemaps, so I decided to take a chance at building a rollup config myself. Here is my attempt. 😅

If you don't want sourcemaps, I think we can stick with microbundle.

However, to fix #46, I believe we'll need this plugin and I don't if it's possible and/or how to add it to microbundle configuration.

PS: I 'm pretty sure this config can be improved, I just don't know how at the moment. If you know how, please let me know. First time using rollup here. 😅

[coveralls]

Coverage remained the same at 96.101% when pulling c3e1148 on arthurdenner:adding-rollup-to-build-process into cb4daee on mweststrate:master.

[coveralls]

Coverage remained the same at 96.101% when pulling b2d42b9 on arthurdenner:adding-rollup-to-build-process into cb4daee on mweststrate:master.

[coveralls]

Coverage remained the same at 96.101% when pulling a420176 on arthurdenner:adding-rollup-to-build-process into cb4daee on mweststrate:master.

[mweststrate]

Thanks! will review in a few days! First want to fix the code reuse between the two immer implementations

[mweststrate]

I have another PR with probably some conflicting changes, so gonna merge this one, and process the final changes there. Thanks for the PR!