LineGauge: gauge_style is confusing

[Fabus1184]

Right now the LineGauge::gauge_style.bg is actually applied to the foreground of the filled part of the line.

This is rather confusing, I would suggest using two different styles (e.g. filled_style, empty_style) for the two parts. This also enables to use different background colors, which are currently always None.

https://github.com/ratatui-org/ratatui/blob/61533712be57f3921217a905618b319975f90330/src/widgets/gauge.rs#L276-L287