feat(rive): stateChange event for state machine transitions

[stasdav]

Title
Add stateChange event for RiveView (iOS + Android)

Summary
Exposes State Machine transitions from the native Rive runtimes to JS.
Works the same on both platforms.

Changes

• common.ts – adds missing stateChangedEvent constant.
• iOS RiveStateMachineDelegateImpl – calls owner.notify({ eventName: 'stateChange', ... }).
• Android listener – mirrors the same call.
• index.d.ts – declares on('stateChange', ...) for type safety.

Why
Apps need to react when a Rive State Machine enters a new state (e.g. to chain logic or UI).
Until now this info was only available inside native code.
No breaking changes; existing code keeps working.

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Stas.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email [email protected]
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[stasdav]

I can't fully test this patch in a real app because npm doesn't support installing a single package from a monorepo subfolder via a GitHub URL. So I can't use my app's package.json to point to this fork or PR branch.

The only way is to build and pack the package locally, or ask a maintainer to publish a prerelease. If there’s a better workflow for app-level testing with monorepo packages, please let me know!

[NathanWalker]

This is excellent @stasdav 💯 - Build/pack is a good way to confirm in a monorepo. npm start > buildrive ENTER, then cd dist/packages/rive > npm pack and just pwd to include full abs path to the .tgz in the monorepo (clean/run).

"@nativescript/rive": "file:/Users/me/path/to/ui-kit/dist/packages/rive/nativescript-rive-x.x.x.tgz"