Support 256 ANSI colour set

[kirawi]

https://docs.rs/crossterm/0.21.0/crossterm/style/enum.Color.html#variant.AnsiValue
Like we have for the base 16 colours:

helix/helix-view/src/graphics.rs

Lines 215 to 237 in e216e96

	#[derive(Debug, Clone, Copy, PartialEq)]
	#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
	pub enum Color {
	Reset,
	Black,
	Red,
	Green,
	Yellow,
	Blue,
	Magenta,
	Cyan,
	Gray,
	LightRed,
	LightGreen,
	LightYellow,
	LightBlue,
	LightMagenta,
	LightCyan,
	LightGray,
	White,
	Rgb(u8, u8, u8),
	Indexed(u8),
	}

[kirawi]

Wait, never mind it's already there.

[chtenb]

Is this feature actually documented anywhere? I'm trying to figure out how to use the 256 colors in a theme, and almost created an issue for it when I stumbled upon this thread.

[the-mikedavis]

Parsing for this was added in #5119 (after the 22.05 release, so you will need to run on master). It looks like there aren't any docs for it yet.

[chtenb]

I see, thanks!