Bump React dependency and peer dependency to 17

[abstracthat]

React 17 is out. I've tested this in a large Karet application and found no issues. This release will mostly just clear the npm warning for the React peer dependency.

[polytypic]

Thanks!

One issue is that tests are still run with 16.* due to the devDependencies specifying "react": "^16.8.0".

[abstracthat]

I had that change locally but missed pushing it. Thanks @polytypic!

[polytypic]

Hmm... One more issue came to mind. Namely that since this is upgrading React major version from 16 to 17, then it would probably be best to also change the major version number of Karet (4.1.0 -> 5.0.0). That is because React is effectively an interface dependency that leaks through. Other than that this looks good!

[mAAdhaTTah]

If you make the peerDep version something like ^16.8.0 || ^17.0.0, consumers can choose which major version they want to use and your package indicates that it works with both, so it wouldn't be a breaking change for people still on React 16.

[bpinto]

ping @abstracthat