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!');

Comienza en el punto de entrada util.styleText y lee el manejo existente de colores hexadecimales para las secuencias SGR de primer plano. Amplía el comportamiento para la forma con el prefijo bg y verifica que los estilos de primer plano y de fondo se combinen mediante la sintaxis de array. Está hecho cuando bg#ff5733 produce la secuencia 48;2;r;g;b, mientras que #ffffff más bg#ff5733 produce ambos estilos.