Add support for child menues in matheader

[piranna]

What happened?

• Minimal Mistakes version: 4.24.0
• Ruby gem or remote theme version:
• Jekyll version:
• Git repository URL:
• Hosted on GitHub Pages (if yes provide URL to site):
• Operating system:

Expected behavior

When adding children field in main navigation, it should create sub-menues, but they are being ignored.

Steps to reproduce the behavior

I think never worked, seems to be a missing feature.

Other

No response