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!');

从 util.styleText 入口开始，阅读现有的前景 SGR 序列十六进制颜色处理。将行为扩展到带 bg 前缀的形式，并验证前景和背景样式可通过数组语法组合。完成标准是 bg#ff5733 生成 48;2;r;g;b 序列，而 #ffffff 加上 bg#ff5733 生成两种样式。