util.styleText: support background colors with hex codes

What is the problem this feature will solve?

util.styleText() supports arbitrary foreground colors via hex codes (styleText('#ff5733', text), added in #61556), but there's no equivalent for background colors. Predefined colors already have both a foreground (red) and background (bgRed) variant, but hex colors only cover the foreground case (SGR 38;2;r;g;b).

What is the feature you are proposing to solve the problem?

Extend hex color detection in styleText to also accept a background variant, mirroring the existing bgRed/red naming convention by prefixing the hex code with bg:

styleText('bg#ff5733', 'Warning!');

This would emit the background TrueColor SGR sequence (48;2;r;g;b) instead of the foreground one (38;2;r;g;b), and composes with the existing array syntax to combine foreground + background:

styleText(['#ffffff', 'bg#ff5733'], 'Warning!');

Beginne am Einstiegspunkt util.styleText und lies die bestehende Behandlung von Hex-Farben für Vordergrund-SGR-Sequenzen. Erweitere das Verhalten für die Form mit dem Präfix bg und überprüfe, dass Vordergrund- und Hintergrundstile über die Array-Syntax kombiniert werden. Erledigt ist es, wenn bg#ff5733 die Sequenz 48;2;r;g;b erzeugt, während #ffffff plus bg#ff5733 beide Stile erzeugt.