Fill in missing bright/dim colors for user-provided color schemes

[boomshroom]

If someone's color scheme comes from base16 or base24, then it may not have proper support for bright or dim colors. Luckily, Cosmic Term has support for modifying the base colors to form bright and dim variants, however this is only used to provide dim colors for the built-in dark and light color schemes, and the bright variant isn't used at all.

As it stands, missing colors in the color scheme default to pure black, which isn't terribly useful, and copying the regular colors unchanged would make more sense, even if proper dim and bright colors would be preferable.

Cosmic term:

Foot (bright colors other than white and black copied from corresponding regular colors):

The color scheme in the screenshots is Atelier Forest, which only supports base16's 8 hues and 8 shades.