Throw exception when adding layer with duplicate ID

[1ec5]

Similar to #7011, we should throw an exception when adding a layer to the style whose ID is already in use by another layer in the style. The style specification describes the id property on a layer as “unique”, and mbgl::Map::getLayer() relies on that fact.

Alternatively, it would be possible to silently replace the existing layer that has a matching ID. But I’d prefer that we make the developer explicitly replace the layer, since reusing an ID is most likely an unintentional error.

/cc @ivovandongen @jfirebaugh