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!');

エントリーポイントの util.styleText から始め、前景 SGR シーケンスに対する既存の 16 進カラー処理を読みます。bg プレフィックス形式にも動作を拡張し、配列構文によって前景スタイルと背景スタイルが合成されることを確認します。bg#ff5733 が 48;2;r;g;b シーケンスを生成し、#ffffff と bg#ff5733 の組み合わせが両方のスタイルを生成すれば完了です。