Listbox not possible to mix group and non-group options

[gimler]

Describe the bug

It is not possible to mix grouped and non-grouped values. In a normal htlm selectbox this is possible.

Pull Request Link

No response

Reason for not contributing a PR

• Lack of time
• Unsure how to implement the fix/feature
• Difficulty understanding the codebase
• Other

Other Reason

No response

Reproducer

https://stackblitz.com/edit/4g2zwyb2?file=src%2FApp.vue

Environment

stackblitz

Vue version

3.2.45

PrimeVue version

4.2.5

Node version

No response

Browser(s)

No response

Steps to reproduce the behavior

see example

Expected behavior

it is possible to mix grouped and non-grouped values

[github-actions]

Due to PrimeTek's demanding roadmap for PrimeVue and the limited bandwidth of the core team, this issue is available for anyone to work on. Make sure to reference this issue in your pull request. ✨ Thank you for your contribution! ✨