Support external model integration in @yorkie-js/react

[hackerwins]

Description:

We recently developed @yorkie-js/react to provide React bindings for Yorkie.
Currently, users receive a yorkie.Document via DocumentProvider and treat it as the source of truth(SOT).

However, in many real-world cases, editors(e.g., tldraw, Slate, Quill) maintain their own model as SOT instead of relying directly on yorkie.Document(details: document-editing.md). In such scenarios, the following workflow needs proper support:

1. External model → yorkie.Document: When local edits occur in the editor, they should be reflected in the yorkie.Document.
2. yorkie.Document → External model: When remote changes occur in the yorkie.Document, the external editor model should be updated accordingly.

At the moment, @yorkie-js/react does not provide a clear interface for this two-way synchronization, which leads to boilerplate and makes integration harder.

Improvement ideas (to be researched further):

• Provide an adapter or binding API that connects an external model with a yorkie.Document.
• Expose hooks (e.g., useDocumentBinding) that encapsulate common synchronization patterns.
• Experiment with migrating the react-tldraw example from @yorkie-js/sdk to @yorkie-js/react to validate this approach.

Next steps:

• Research possible designs for external model bindings.
• Explore trade-offs between flexibility and ease of use.
• Use react-tldraw as a test case for validating the integration pattern.

Why: