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

[alamenai]

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():

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