New Zed black theme
Big theming refactor, fixes #82

All themes are now managed by Zed itself and reside in the Configuration
project (see themes.json and /default/theme/*). So it's now possible to
create your themes from within Zed (although changes to the CSS will
require a reopening of the editor).

As a bonus: new custom Zed Dark theme, which is the default.

Existing users: your theme will be reset to "Zed Dark" initially since
the theme names changed.