Use esbuild for bundling

[lindskogen]

This is an initial draft for a solution to #953.

esbuild doesn't seem to support UMD builds which could be a dealbreaker.

Also, there's nothing like rollup-plugin-strip-code for esbuild yet.

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[markerikson]

Hey there, thanks for the PR! Appreciate you filing this. Unfortunately, there's actually already an in-progress PR to convert our build system over to ESBuild over in #957 , which is close to complete. I was actually just about to review it again to see if there's anything left to be done.

Given that, I'm afraid this probably needs to be closed. Sorry about that! However, I'd definitely appreciate any help you could offer with some of the other open issues we're tracking for our 1.6 milestone:

https://github.com/reduxjs/redux-toolkit/milestone/3