Now that the decision not to support colors on Linux
has been made, we can replace the custom `Color` struct
with simple typealiases for `NSColor` and `UIColor`.
The same can’t be done for font, since we want to enable
loading of a font from a path, for SPM packages.
Now that we’re making Splash support iOS as well as
Mac + Linux, we need to create some nice abstractions
to make sure that we can share as much code as possible
between all platforms.
- Define Font.Loaded and Color.Renderable as platform-
specific typealiases for system fonts and colors.
- Don’t compile in non-Linux compatible code when building
for Linux.
- Make Font and Color handle all conversion themselves, so
that AttributedStringOutputFormat can be kept more clean.
