[refactor] move isValidElementType to react-is

[rickhanlonii]

After owner stacks, we don't check this in ReactJSX, so it's really only used in react-is and memo. Neither of these are really accurate long term, because renderers can define their own element type.

So for memo I'm just changing it to a nullish check, which will allow custom renderer elements to be wrapped in memo, so we can move this to react-is so nothing internally starts using it.

[react-sizebot]

Comparing: e0fe347...972d226

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	+0.05%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	518.32 kB	518.32 kB	=	92.45 kB	92.45 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	+0.05%	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	573.25 kB	573.25 kB	=	102.08 kB	102.08 kB
facebook-www/ReactDOM-prod.classic.js	=	641.70 kB	641.70 kB	=	112.86 kB	112.86 kB
facebook-www/ReactDOM-prod.modern.js	=	632.02 kB	632.02 kB	=	111.30 kB	111.30 kB
facebook-www/React-dev.classic.js	=	55.41 kB	54.22 kB	=	12.07 kB	11.92 kB
facebook-www/React-dev.modern.js	=	55.41 kB	54.22 kB	=	12.07 kB	11.92 kB
oss-experimental/react/cjs/react.react-server.development.js	=	35.90 kB	34.99 kB	=	8.47 kB	8.35 kB
oss-stable/react/cjs/react.react-server.development.js	=	28.86 kB	28.01 kB	=	6.94 kB	6.81 kB
oss-stable-semver/react/cjs/react.react-server.development.js	=	28.84 kB	27.98 kB	=	6.91 kB	6.78 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
facebook-react-native/react/cjs/React-dev.js	=	51.03 kB	50.23 kB	=	11.36 kB	11.25 kB
oss-experimental/react/cjs/react.development.js	=	46.95 kB	46.11 kB	=	10.65 kB	10.53 kB
oss-stable/react/cjs/react.development.js	=	45.21 kB	44.35 kB	=	10.28 kB	10.15 kB
oss-stable-semver/react/cjs/react.development.js	=	45.18 kB	44.33 kB	=	10.26 kB	10.13 kB
facebook-www/React-dev.classic.js	=	55.41 kB	54.22 kB	=	12.07 kB	11.92 kB
facebook-www/React-dev.modern.js	=	55.41 kB	54.22 kB	=	12.07 kB	11.92 kB
oss-experimental/react/cjs/react.react-server.development.js	=	35.90 kB	34.99 kB	=	8.47 kB	8.35 kB
oss-stable/react/cjs/react.react-server.development.js	=	28.86 kB	28.01 kB	=	6.94 kB	6.81 kB
oss-stable-semver/react/cjs/react.react-server.development.js	=	28.84 kB	27.98 kB	=	6.91 kB	6.78 kB

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