BFormSelect turns options into groups if a label field is in it

### Describe the bug

Since 0.26.0, BFormSelects turns options into groups if a label field is in it. Reason is that the option is spread into the normalized option but isComplex checks if the `label` field is in the option.

### Reproduction

https://stackblitz.com/edit/github-ro4xihmw?file=src%2FApp.vue

### Used Package Manager

npm