add colored attributes: tint.Attr(color, attr)

[lmittmann]

This PR adds the function tint.Attr to easily write colored logs:

func Attr(color uint8, attr slog.Attr) slog.Attr

Attr returns a tinted (colorized) slog.Attr that will be written in the
specified color by the [tint.Handler]. When used with any other [slog.Handler], it behaves as a
plain slog.Attr.

Use the uint8 color value to specify the color of the attribute:

• 0-7: standard ANSI colors
• 8-15: high intensity ANSI colors
• 16-231: 216 colors (6×6×6 cube)
• 232-255: grayscale from dark to light in 24 steps

See https://en.wikipedia.org/wiki/ANSI_escape_code#8-bit

Example:

Create a new logger with a custom TRACE level:

const LevelTrace = slog.LevelDebug - 4

w := os.Stderr
logger := slog.New(tint.NewHandler(w, &tint.Options{
    Level: LevelTrace,
    ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
        if a.Key == slog.LevelKey && len(groups) == 0 {
            level, ok := a.Value.Any().(slog.Level)
            if ok && level <= LevelTrace {
                return tint.Attr(13, slog.String(a.Key, "TRC"))
            }
        }
        return a
    },
}))

resolves #61