refactor: Refactor model initialization logic

[kylebarron]

In ipywidgets, a "model" is the core of reactivity. In particular, a Lonboard Map is reactive, but every underlying layer is also reactive, so that when a user changes any property on a layer, it automatically gets updated on the map.

By extension, our Extension classes are also themselves models. These are separate Python classes that get serialized as their own reactive models on the JS side.

Soon we'll have more types of widget models that we want to be reactive, including views and basemap styles (#908).

Right now, model initialization is slightly hard-coded to handle layer initialization. This PR refactors the model initialization to be fully generic.

[Copilot]

Pull Request Overview

This PR refactors model initialization logic to be fully generic instead of being hardcoded for layer initialization, preparing for future support of additional reactive widget models like views and basemap styles.

• Moved generic model loading logic from util.ts to a new dedicated model/initialize.ts file
• Created a generic initializeChildModels function that can handle any type of child model with proper lifecycle management
• Updated layer initialization to use the new generic infrastructure while maintaining existing functionality

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/util.ts	Removed layer-specific loadChildModels function to consolidate model loading logic
src/model/initialize.ts	Added new generic model initialization utilities with proper lifecycle management
src/model/index.ts	Exported the new initializeChildModels function for external use
src/model/base-layer.ts	Updated to use generic model initialization and changed extensions from array to Record
src/index.tsx	Refactored to use the new generic model initialization infrastructure

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[kylebarron]

Open to comments still, but this is a pretty straightforward refactor I think, so I'll merge so that I can use it in #935