[React Native] Add react-native.config.js and Expo plugin MainApplication patch to fix autolinking

[dccarmo]

Summary

This PR fixes the long-standing issue where onnxruntime-react-native requires
manual native setup after installation, causing the TypeError: Cannot read property 'install' of null runtime crash.

Root cause: The package shipped without a react-native.config.js, so the RN
community CLI autolinking did not register OnnxruntimePackage on Android. For Expo
users on the New Architecture, the existing app.plugin.js patched Gradle and the
Podfile but never registered the package class in MainApplication.

Changes:

• react-native.config.js (new) — enables RN community autolinking for Android.
  With RN 0.74+ this covers both old and new architecture via the generated
  PackageList.java. No manual settings.gradle or MainApplication edits needed
  for bare RN.

• app.plugin.js — adds a withMainApplication mod that idempotently inserts
  the OnnxruntimePackage import and registration into MainApplication.kt/.java
  during expo prebuild. Uses the same mergeContents tag pattern as the existing
  Gradle/Podfile mods, so it is safe to run multiple times.

• package.json — includes react-native.config.js in the npm files array so
  it ships in the tarball.

• README.md — documents that autolinking handles registration automatically and
  adds the Expo plugin usage snippet.

Testing

• Bare RN: npx react-native config | jq '.dependencies["onnxruntime-react-native"]'
  should show packageImportPath populated; PackageList.java should include
  OnnxruntimePackage after a Gradle sync.
• Expo: npx expo prebuild --clean should produce a MainApplication.kt containing
  import ai.onnxruntime.reactnative.OnnxruntimePackage and add(OnnxruntimePackage()).

Fixes #19510
See also #17773

[Copilot]

Pull request overview

This PR aims to fix Android (and Expo prebuild/New Architecture) autolinking for onnxruntime-react-native to avoid runtime crashes caused by the native package not being registered.

Changes:

• Add react-native.config.js to explicitly provide Android autolinking metadata (packageImportPath / packageInstance).
• Extend the Expo config plugin (app.plugin.js) to patch MainApplication.kt/.java with OnnxruntimePackage import + registration.
• Ensure the new config file is published (package files array) and document expected setup in README.md.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
js/react_native/react-native.config.js	Adds explicit RN CLI autolinking configuration for Android.
js/react_native/app.plugin.js	Adds Expo prebuild MainApplication patching to register OnnxruntimePackage.
js/react_native/package.json	Ships react-native.config.js in the published package.
js/react_native/README.md	Documents autolinking behavior and Expo plugin usage.

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[dewana-sl]

@dccarmo please accept the license cla. I hope this gets merged soon

[dccarmo]

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