"mapLayer.layout.get()" is no longer a function

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**mapbox-gl-js version**: 3.5.2

**browser**: Safari, Chrome, Firefox

### Steps to Trigger Behavior

1. attempt to use `map.getLayer(LAYER_ID).layout.get('icon-size').value.value`

### Link to Demonstration

### Expected Behavior

The value of the layer property is returned.

### Actual Behavior

An error stating that get is not a function (it is undefined) is thrown.

### Other tech details

I found that the computed value is now available under `map.style._layers[LAYER_ID].layout._values['icon-size'].value.value`, but this is both using what looks like an internal value, and the types are not set up well to use like `.get()` was with the `@types/` library previously.

Also, `map.style._layers[LAYER_ID]` has a type of `StyleLayer`, which is a base type, (rather than being `TypedStyleLayer` which is the combined type), so there’s no way to use type guards to narrow it to the type, (`SymbolStyleLayer`), required.

Reproduce the failure from map.getLayer(LAYER_ID).layout.get('icon-size') and compare it with the internal map.style._layers[LAYER_ID].layout._values access described in the report. Inspect the related style-layer types and the @types/ usage; done means the layer property can be retrieved through a supported, type-safe API without relying on internal values.

Written by the indexing model from the issue text.