Adding react is export support

[mrgurdeep]

Adding support so that each function of ReactIs is individually accessible

This solve the issue mentioned in #13250

And then finally provide a feature requested in #13272

What is the issue
Tree shaking not enabled with react-is.

import * as ReactIs from "react-is";
ReactIs.isValidElementType(<div />); // true

Expected behavior
Allow importing individual modules from the package.

import { isValidElementType } from "react-is";
isValidElementType(<div />); // true

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[pull-bot]

Details of bundled changes.

Comparing: 840cb1a...4dc0620

react-is

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react-is.development.js	+12.6%	+10.1%	4.68 KB	5.28 KB	1.29 KB	1.42 KB	UMD_DEV
react-is.production.min.js	🔺+18.8%	🔺+20.7%	1.89 KB	2.25 KB	789 B	952 B	UMD_PROD
react-is.development.js	+13.1%	+11.2%	4.5 KB	5.09 KB	1.24 KB	1.38 KB	NODE_DEV
react-is.production.min.js	🔺+17.5%	🔺+18.7%	1.84 KB	2.16 KB	722 B	857 B	NODE_PROD
ReactIs-dev.js	+12.9%	+10.8%	4.58 KB	5.17 KB	1.27 KB	1.4 KB	FB_WWW_DEV
ReactIs-prod.js	🔺+24.8%	🔺+17.9%	3.68 KB	4.59 KB	1006 B	1.16 KB	FB_WWW_PROD

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