fix: ANSI background colors not rendered correctly

[gabe565]

Currently, ANSI background colors are rendered as either gray or black since the fill is not correctly selected. This PR makes the correct color get chosen.

Before:

After:

I generated the previews with gabe565/print-xterm256-go if you want to test this change.

[bashbunni]

Hey, why do we need the -10 here?

[aymanbagabas]

I believe to account for the color indices in https://github.com/charmbracelet/freeze/blob/main/ansi.go#L190. Since 40s and 100s are the same ANSI16 colors but for background, we need to -10 to get the correct colors from ansiPalette

[gabe565]

Yep it's because the background and foreground colors are the same, but the background codes are shifted up by 10.

[meowgorithm]

Why don't we pull out this magic number, i.e const backgroundColorOffset = -10

[aymanbagabas]

We could also normalize the ansiPalette to use indices from 0-15 which imo makes more sense

[meowgorithm]

Totally, that's way clearer

[gabe565]

I like that idea! Or maybe something like this?
Add a separate ansiPaletteBright:

var ansiPalette = [8]string{
	"#282a2e", // black
	"#D74E6F", // red
	"#31BB71", // green
	"#D3E561", // yellow
	"#8056FF", // blue
	"#ED61D7", // magenta
	"#04D7D7", // cyan
	"#C5C8C6", // white
}

var ansiPaletteBright = [8]string{
	"#4B4B4B", // bright black
	"#FE5F86", // bright red
	"#00D787", // bright green
	"#EBFF71", // bright yellow
	"#8F69FF", // bright blue
	"#FF7AEA", // bright magenta
	"#00FEFE", // bright cyan
	"#FFFFFF", // bright white
}

For foreground colors:

case 30, 31, 32, 33, 34, 35, 36, 37:
	span.CreateAttr("fill", ansiPalette[v%10])
	p.lines[p.row].AddChild(span)
case 90, 91, 92, 93, 94, 95, 96, 97:
	span.CreateAttr("fill", ansiPaletteBright[v%10])
	p.lines[p.row].AddChild(span)

For background colors:

case 40, 41, 42, 43, 44, 45, 46, 47:
	p.beginBackground(ansiPalette[v%10])
case 100, 101, 102, 103, 104, 105, 106, 107:
	p.beginBackground(ansiPaletteBright[v%10])

I have verified this still renders the correct colors.

(Defining these as arrays is obviously unnecessary, but I opted to do that to make it clear that these only support 8 colors. I'd be fine with just using []string if that's what you all prefer)