[Feature Request]: Inverted group label fg color for transparent themes

### What?

Use an inverted fg color for group label when using transparent background or provide a way to specify the group label fg color in the the group highlights.

### Why?

Currently it seems that the group label fg color is always set to `fill.bg`(see groups.lua) when group highlight is specified. When using transparent background in my theme (cappuccin), the text is unreadable (bright instead of dark )

As a workaround I have to override the BufferLine_GroupName_Label highlight color manually.