Remove index.js suffix of use-sync-external-store/shim to support React Native

[malash]

This PR fixed #2402

According to this document, React Native-specific code can use the '.native.js' suffix, while standard React web code uses the '.js' suffix. By removing index.js suffix of use-sync-external-store/shim, bundling tools like Metro/Webpack could choose the right file for each platform.

I haven't tested this PR on React Native because I'm not familiar with React Native development and I have no idea how to downgrade React to 17. However, I can reproduce the issue and test the PR on GojiJS, which is a React-like framework based on react-reconciler and shares many technical specifications with React Native, including the '.native.js' suffix. It would be appreciated if anyone could help verify the changes.

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Secrets	0   0   0   0	View in Orca

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[promer94]

Thank you!

[maziar-dandc]

This has broken our prod builds, reproduced on node v14, v16.
Going to pin SWR to an older version for the time being.
If this is caused by a problem from our side, please feel free to point it out, I'm not a FE expert by any means.

[14:57:16] Error in plugin "webpack-stream"
Message:
    Module not found: Error: Can't resolve 'use-sync-external-store/shim' in '/home/~~~~/node_modules/swr/core/dist'
Did you mean 'index.js'?
BREAKING CHANGE: The request 'use-sync-external-store/shim' failed to resolve only because it was resolved as fully specified
(probably because the origin is strict EcmaScript Module, e. g. a module with javascript mimetype, a '*.mjs' file, or a '*.js' file where the package.json contains '"type": "module"').
The extension in the request is mandatory for it to be fully specified.
Add the extension to the request.
Did you miss the leading dot in 'resolve.extensions'? Did you mean '[".*",".js",".jsx"]' instead of '["*",".js",".jsx"]'?
Details:
    domainEmitter: [object Object]
    domainThrown: false

[14:57:16] 'default' errored after 21 s