feat: Add option to render concurrent roots

[eps1lon]

First draft. There's a lot to figure out (like error message if the experimental build isn't used or if this cleanup is correct etc)

What:

Adds root option to render which currently only allows concurrent but could be used to create blocking roots instead

Why:

hype-driven-development

How:

Added a lot of branching logic but overall the implementation would be simpler with create*Root.

Checklist:

• Documentation added to the
  docs site
• Tests
• Typescript definitions updated
• Ready to be merged
  

[eps1lon]

Biggest unknown to figure out. I think we need to use this to advance time (or something like that) if we have tests scheduling work (like setState).

[danielkcz]

I wonder why are tests failing to find createRoot method. The package.json is correctly updated, there is no lock file to update, so it's rather weird.

[kentcdodds]

I think that this resolves to the latest stable version. If we want to support the experimental release we have to do an exact version rather than ^. I believe that's why tests are failing.

[kentcdodds]

I'm not sure how close concurrent mode is (but I get the impression that it's not very close) so I think we should just close this for now and open it when concurrent mode is closer.

Thanks!

[eps1lon]

Yeah that was me playing around. Not sure how testing will look with a mocked scheduler.