Handle browser code at build time (vs runtime)

[benmccann]

Fixes https://github.com/pubkey/broadcast-channel/issues/636

I had to include #678 in this PR to be able to get anything done. If you'd prefer to merge that one first, I can rebase this one afterwards to drop those changes out of this PR

jsnext:main has been deprecated in favor of module so remove it
browser refers to the client-side entry point: https://docs.npmjs.com/cli/v7/configuring-npm/package-json#browser
main has to not use import and also provide the Node method, so I had to create a new build for that

Calling Babel a bunch of times from package.json feels a bit messy to me compared to having a single Rollup config that can output different builds. Rollup would also allow us to use @rollup/plugin-replace instead of sed for removing the Node method. However, this is a bigger change, so didn't want to do that as part of this PR

[pubkey]

I merged #678
The code here looks good. But I noticed in the CI that this increased the build size dramatically.
For example the webpack build size goes from 4727 to 5479
Can you check if the node method is really correctly excluded from the builds?

[benmccann]

Thanks for pointing that out. The reason webpack is bigger now is because I pointed it at the babelified version. I can verify that the node method is correctly excluded (I both see the code missing and verified that the webpack build fails if it were included)

Is the size increase an issue? If so, the best solution I think would be to not run the code through babel. Typically the end-developer should be responsible for this and not the library. E.g. for developers that need to only support newer browsers and are using Browserify today, they are getting a larger bundle than necessary because they are getting the Babelified version. Also, if a developer wants to support older browsers, they are typically going to have to setup Babel regardless (both for their own code and other libraries), so most of the time it's no extra work for them anyway

[pubkey]

Merged.
I am ok with pointing to the babeled version for now.
Thank you for the work, I will create a new release today.