Support 24-bit color

[noelwelsh]

There are three ways to specify colors in the terminal:

• a handful of named colours. Terminus supports this already
• 256 (i.e. 8-bit) color.
• 24-bit color

There doesn't seem to be any point in supporting 8-bit color when 24-bit color is available.

We should support specifying 24-bit colors for foreground, background, underline, and any other place where terminals support it. Consider reusing or copying the Color type from Doodle, which will also provide more convenient HSL specification and many predefined colors.

[noelwelsh]

The 0.28.0 release of Doodle has better color support. We can just depend on the doodle-core module.

[noelwelsh]

This is currently blocked by Doodle not being available on native, which in turn is blocked by Cats Effect not being available on Scala Native 0.5.