Fix basemaps order

[glughi]

What this PR does

Fixes #7169

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[zoran995]

this.enabledBaseMaps might be undefined, and when undefined it should show all base maps available.
i.e. simple sorting before return in the old code would do the trick

// Sort based on enabledBaseMaps order
if (this.enabledBaseMaps) {
  enabledBaseMaps.sort((a, b) => {
    const aIndex = this.enabledBaseMaps!.indexOf(a.item.uniqueId!);
    const bIndex = this.enabledBaseMaps!.indexOf(b.item.uniqueId!);
    return aIndex - bIndex;
  });
}

another option that comes to my mind is to go with the utility function and do mapping over proper array

[glughi]

@zoran995 what do you think about

    const baseMapList =
      this.enabledBaseMaps ?? this.items.map((item) => item.item);
    baseMapList.forEach((baseMapItem) => {
      const item = this.items.find((item) => item.item === baseMapItem);
      if (item && !ModelReference.isRemoved(baseMapItem)) {
        const itemModel = this.terria.getModelById(BaseModel, baseMapItem);
        if (MappableMixin.isMixedInto(itemModel)) {
          enabledBaseMaps.push({
            image: item.image,
            contrastColor: item.contrastColor,
            item: itemModel
          });
        }
      }
    });

[zoran995]

on first look, it should work correctly and the rest is a personal preference on the code structure. It would be great to cover sorting with the unit test to avoid regression bugs in future, there are tests for BaseMapsModel

https://github.com/TerriaJS/terriajs/blob/2490260c78e4b7666496bfc6d0ec677eac63a98c/test/Models/BaseMaps/BaseMapsModelSpec.ts