Visual Studio Code Dark Theme

Based on Dark+ (the default VS Code Dark Theme), Sable seeks to improve the legibility of code through enhanced color coding while simultaneously minimizing eye-strain.

While the current development focus is on improving Go syntax highlighting, I will gladly accept PRs for any other language.

Note: For visibility in the theme selection popup, the theme was given the label Happy Ferret's Sable (Dark) rather than Sable (Dark). See also this 2019 GDC talk.