(v2) Adaptive standalone

[meowgorithm]

This PR reworks the adaptive color workflow to be more explicit in standalone mode (in the spirit of the new Lip Gloss writer). It includes @bashbunni's suggestion to rename LightDark to Adapt.

// Detect the background color, explicitly choosing inputs and outputs.
hasDarkBG, err := lipgloss.HasDarkBackground(os.Stdin, os.Stdout);
if err != nil {
    fmt.Fprintf(os.Stderr, "Dang: %v", err)
    os.Exit(1)
}

// Use lil' adaptive colors.
adaptive := lipgloss.Adapt(hasDarkBG)
myHotColor := adaptive.Color("#ff0000", "#0000ff")

Supersedes #392.

[meowgorithm]

This is my fault, but this is a PR for the same branch as #397. Closing accordingly.