Devtools: Streamline getting extension from branch

[eps1lon]

Stack:

1. Devtools: Streamline getting extension from branch #28975 <--- You're here
2. Devtools: Fix build-for-devtools #28976
3. DevTools: Fix inspecting components with multiple reads of the same Context in React 17 #28974

Summary

For manual e2e testing, I oftentimes just need the extension for the browser I'm using. So I'm adding separate artifacts just for Chrome and Firefox extension instead of a single tarball with both. Avoids one unpack step and is also easier for external people to grab a nightly build of the extension.

How did you test this change?

• Download react-devtools-chrome-extension.zip from build_devtools_and_process_artifacts artifacts:

[react-sizebot]

Comparing: 4508873...ba75d2d

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	=	6.66 kB	6.66 kB	=	1.82 kB	1.82 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.67 kB	6.67 kB	=	1.83 kB	1.83 kB
facebook-www/ReactDOM-prod.classic.js	=	591.11 kB	591.11 kB	=	103.94 kB	103.94 kB
facebook-www/ReactDOM-prod.modern.js	=	567.33 kB	567.33 kB	=	100.34 kB	100.34 kB
test_utils/ReactAllWarnings.js	Deleted	64.26 kB	0.00 kB	Deleted	16.02 kB	0.00 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
test_utils/ReactAllWarnings.js	Deleted	64.26 kB	0.00 kB	Deleted	16.02 kB	0.00 kB

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