Define JSI bridging function for ShadowNode::Shared (#51675)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51675

Changelog: [internal]

This defines a new bridging method to convert ShadowNode::Shared from and to JS. This has some benefits over the existing `shadowNodeFromValue` and `valueFromShadowNode` functions:
1. It's more convenient, as we can just use `ShadowNode::Shared` in classes implementing the codegen, and the runtime will do the conversion automatically.
2. It's safer, as it checks for nullability and throws JS exceptions except of having segmentation faults down the line.

Reviewed By: javache

Differential Revision: D75597542

fbshipit-source-id: 37c0c43cbde0f4de9a740d9da910f215ae50efbd