feat: add viewModelName param to useViewModelInstance

[mfazekas]

Summary

• artboardName and viewModelName are now mutually exclusive (TypeScript discriminated unions)
• Clarified docs: "ViewModel assigned to artboard" instead of implying ownership

API

// RiveFile source - pick ONE of: default, artboardName, or viewModelName
useViewModelInstance(riveFile, {
  instanceName?: string       // which instance (default if omitted)
})
// OR
useViewModelInstance(riveFile, {
  artboardName: string,       // get ViewModel assigned to this artboard
  instanceName?: string
})
// OR
useViewModelInstance(riveFile, {
  viewModelName: string,      // get ViewModel by name (file-wide)
  instanceName?: string
})

// ViewModel source (unchanged)
useViewModelInstance(viewModel, {
  name?: string,
  useNew?: boolean
})

[HayesGordon]

LGTM! We'll need to update the docs to reflect this as well.