Add babel plugin for rollup & transform code

[etimberg]

Adds the babel rollup plugin to enable moving the codebase to ES6+.

I did not configure the preset targets yet. We can do that once we decide what exactly we want to support.

[benmccann]

@etimberg I don't have any major objections to this since we already need to compile the code to run it, but I know that @simonbrunel had wanted to wait until v3. See #5787 (comment)

[etimberg]

Perhaps we should leave this until after 2.9 and then make the next version v3? Seems like we've got a long list of stuff to do

[RobbieTheWagner]

Very much in favor of adding babel. There should be no reason we could not enable it now. It should not break anything. If things need no transpiling, it should be a noop.

[etimberg]

@benmccann now that v2.9 is out, shall we merge this?

[benmccann]

I think using a .browserslistrc is preferred over targets. What do you think about adding a .browserslistrc that just uses their defaults for now? I think you just create the file and put the word "defaults" in it.

"defaults" is defined as: https://github.com/browserslist/browserslist/blob/2fee1e4d7b1bae0214bbbef10d04087a2785dfd3/index.js#L429

[etimberg]

@benmccann I added the .browserlistrc file & copied the defaults over.