Add new color theme: VSCodium Dark

Theme is similar to default “Dark” and "Dark+" themes used in VS Code
and VSCodium. "VS Code" is a trademark so this name is used instead.

Not all contexts have same color as original mainly due differences in
code parsing and not all language specific settings are included.
Adding more of these can offer better match for the original. At the
moment, there's definitions mainly for CSS, SCSS, Markdown, Javascript
and Typescript.

Theme also has some other slight enhancements / distinctions from
original:

- Usage of bold and cursive font variants for some contexts.
- Visible separator between same color text area and background.
  Original doesn't have any which makes it hard to see where the actual
  text area starts.
- Kate uses for highlighting of current line a background color instead
  of lines at top and bottom of line.
- Some colors are slightly enhanced when selected to offer better
  contrast.