react-is: Add 16.7 features

[eps1lon]

Changes

• added react-is@16.7 features except anything related to ConcurrentMode since it is not implemented in @types/react yet.
• fixed test assertions concerning forwardRef

Please fill in this template

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change. (Run with npm test.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

Select one of these and delete the others:
If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes:
  • https://github.com/facebook/react/blob/v16.7.0/packages/react-is/src/ReactIs.js
• Increase the version number in the header if appropriate.
• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }.

[typescript-bot]

@eps1lon Thank you for submitting this PR!

🔔 @AviVahl @christianchown - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[AviVahl]

LGTM

[typescript-bot]

A definition owner has approved this PR ⭐️. A maintainer will merge this PR shortly. If it shouldn't be merged yet, please leave a comment saying so and we'll wait. Thank you for your contribution to DefinitelyTyped!

[eps1lon]

@AviVahl @christianchown Thanks for the quick review but I had to change the return type of the isMemo and isLazy typeguards. As the test file confirms they currently do not narrow to ReactElement but ReactComponentType.

See facebook/react#14546 for implementation details