Inline Platform within RN’s Babel preset

[robhogan]

Summary:
Adds a React Native-owned Babel plugin that inlines Platform.OS and
Platform.select(...), from
react-native/babel-preset at the front of the preset's plugin list (importantly, before
the preset lowers ES modules to CommonJS).

Metro already inlines Platform, but its matcher (metro-transform-plugins'
inline-plugin via createInlinePlatformChecks) runs after ESM has been
lowered to CommonJS. By then a genuine import {Platform} from 'react-native'
has become _reactNative.Platform.OS and the specifier proving the value came
from React Native is gone. Metro compensates by matching on the local
identifier spelling - so Platform.OS inlines whether or not Platform is
actually React Native's, and a genuine deep or RN-relative import may be missed entirely.

Platform inlining is in any case much more of an RN concern than a Metro one - it has to be aware of RN APIs and module names, and even the module’s path.

This plugin recognises:

• import {Platform} from 'react-native' (including aliased imports)
• import * as RN from 'react-native', uses of RN.Platform
• import P from 'react-native/Libraries/Utilities/Platform'
• the equivalent require forms, plus destructuring and immutable aliases
• RN's internal relative imports, e.g. ../../Utilities/Platform

…and deliberately does not recognise anything it cannot prove - a bare
Platform.OS global, React.Platform.OS, or a same-named import from another
package.

Metro's late matcher is left fully enabled and remains responsible for
those historical forms, so this change is purely additive: it inlines genuine
imports Metro was missing, and changes nothing Metro already handled. Metro’s plugin will be deprecated and removed in future.

Notable details:

• Relative RN imports are resolved lexically, with no filesystem access and no
  platform-extension resolution — the identity we need is the extension-less
  module <rn-root>/Libraries/Utilities/Platform, and Metro picks
  Platform.ios.js / Platform.android.js later. The RN package root is
  identified by directory name, which covers node_modules/react-native,
  packages/react-native and pnpm layouts alike, and rejects
  react-native-web and friends (who would be expected to adjust the paths in their forks)
• Imports are left in place. Removing them would change dependency collection,
  so it is handled separately.
• A no-platform build (null, or the empty string that RN's Jest preprocessor
  passes) is a no-op — inlining Platform.OS to "" would be wrong.
• The plugin source is added to the preset's getCacheKey so edits invalidate Metro's transform cache.

Changelog:
[General][Changed] - Inline Platform.OS and Platform.select(...) for
React Native Platform imports during the Babel preset, covering some cases that were previously left un-inlined.

Differential Revision: D114647943

[meta-codesync]

@robhogan has exported this pull request. If you are a Meta employee, you can view the originating Diff in D114647943.

[retyui]

@robhogan I guess my PR to Metro doesn't make sense at all react/metro#1732 , right ?

[retyui]

I believe you should avoid adding this plugin in tests (Jest) environment

As devs need to be able to mock Platform.OS value