config: add [colors-dark] and [colors-light], replacing [colors] and [colors2]

The main reason for having two color sections is to be able to switch
between dark and light. Thus, it's better if the section names reflect
this, rather than the more generic 'colors' and 'colors2' (which was
the dark one and which was the light one, now again?)

When the second color section was added, we kept the original name,
colors, to make sure we didn't break existing configurations, and
third-party themes.

However, in the long run, it's probably better to be specific in the
section naming, to avoid confusion.

So, add 'colors-dark', and 'colors-light'. Keep 'colors' and 'colors2'
as aliases for now, but mark them as deprecated. They WILL be removed
in a future release.

Also rename the option values for initial-color-theme, from 1/2, to
dark/light. Keep the old ones for now, marked as deprecated.

Update all bundled themes to use the new names. In the light-only
themes (i.e. themes that define a single, light, theme), use
colors-light, and set initial-color-theme=light.

Possible improvements: disable color switching if only one color
section has been explicitly configured (todo: figure out how to handle
the default color theme values...)

themes/nord

@@ -6,7 +6,7 @@
 # this specific foot theme is based on nord-alacritty:
 # https://github.com/arcticicestudio/nord-alacritty/blob/develop/src/nord.yml
 
-[colors]
+[colors-dark]
 cursor = 2e3440 d8dee9
 foreground = d8dee9
 background = 2e3440