Adding ANSI color constants

[givensuman]

It's unintuitive to write lipgloss.ANSIColor(6) to create cyan. I think creating constants for the ANSI color set would be useful. Example:

lipgloss.ANSIColor(lipgloss.ANSICyan)

Additionally, it might be useful to add another type XTermColors to extend out to 256 color support.

[DaltonSW]

I'm pretty sure something like this already exists with v2

[givensuman]

Yup! Didn't know about v2 but hot dang it has a lot of good stuff.

const (
	Black ansi.BasicColor = iota
	Red
	Green
	Yellow
	Blue
	Magenta
	Cyan
	White

	BrightBlack
	BrightRed
	BrightGreen
	BrightYellow
	BrightBlue
	BrightMagenta
	BrightCyan
	BrightWhite
)

implements this exactly.