add react-native field in package.json

[TomCorvus]

Fix #46

[douglasjunior]

Thank you!

I'm curious why this is needed, I don't remember to see this in the RN documentation.

[TomCorvus]

It seems necessary to define it to differentiate the entry point according to the environment and recommended to be compatible with new arch.
nodejs/node#45367

We can also define exports key since RN 0.72 like this:

  "main": "dist",
  "react-native": "src",
  "exports": {
    ".": {
      "default": "./dist/index.ts",
      "react-native": "./src/index.js"
    },
    "./package.json": "./package.json"
  },

https://metrobundler.dev/docs/package-exports#migration-guide-for-package-maintainers
https://reactnative.dev/blog/2023/06/21/package-exports-support#the-new-react-native-condition

To respect nodeJS >= 12.7.0 convention
https://nodejs.org/api/packages.html#package-entry-points

And supported by Metro since 0.82 version (RN 0.79):
https://reactnative.dev/blog/2025/04/08/react-native-0.79#metro-faster-startup-and-package-exports-support

And keep react-native key for previous versions.

[douglasjunior]

Nice, thank you.