[Feature]: Support for `beforeId` parameter when adding features to control layer positioning

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### Problem

Currently, when adding features to Mapbox GL Draw using `draw.add()`, the draw layers are always added on top of all existing layers in the map. This causes visual flashing when layers need to be re positioned using `map.moveLayer()` after creation, as users briefly see the layers in the wrong position before they're moved.

### Use Case

In our application, we need footprint draw layers to render below roof surface layers but above parcel layers.

Currently, we have to:

1. Add the draw feature (appears on top)
2. Use `map.on('idle', ...)` to detect when layers are added
3. Call `map.moveLayer()` to reposition them

This causes a visible flash as the layers appear on top first, then get moved to their correct position.

### Proposed Solution

Add support for a `beforeId` option when adding features, similar to Mapbox GL JS's `map.addLayer()`:

```ts

draw.add({
type: "Feature",
geometry: footprintGeometry,
properties: { ... }
}, {
beforeId: 'roof-surface-layer-0' // Position draw layers before this layer
})

```

Start by tracing the draw.add() entry point through feature and layer creation, then compare its insertion behavior with Mapbox GL JS map.addLayer() and the current map.moveLayer() workaround. Add coverage showing that an optional beforeId places draw layers correctly without a visible intermediate position; done means the proposed call works for the stated layer-ordering use case.

Written by the indexing model from the issue text.