No core-js in core

[wegry]

Reasons for making this change

Two versions of core-js currently exist in my app because @rjsf/core pulls in @babel/runtime currently. There are two direct imports of core-js that I've opted to pull in pure imports for.

For the rationale of why library code shouldn't include polyfills, this comment
w3ctag/polyfills#6 (comment) explains my rationale. This is a breaking change though, if downstream folks were depending on @babel/runtime being around.

If this is related to existing tickets, include links to them as well. fixes #1601

Recreates PR for #2049

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[epicfaace]

Thanks for this change! Do you mind also adding a documentation page called "3.x upgrade guide" that describes the nature of this change, and also what people should do (what polyfills they should install) if they were depending on @babel/runtime-corejs2 that used to be bundled with @rjsf/core?

[epicfaace]

Just want to clarify here -- is the only solution here that we have to begin transpiling the code with @babel/preset-env? What if we just ran npm install core-js and added import 'core-js'; -- would that also be a solution?

[wegry]

I added a note to that effect.

[epicfaace]

Looks good! Thanks!

[epicfaace]

@wegry Can you merge master into this branch? Then we should be all ready to merge.

[wegry]

Let me give it a shot @epicfaace.