Implement auto_color_scheme for kitten diff

[Covkie]

Is your feature request related to a problem? Please describe.
*-theme.auto.conf currently exists for main kitty but not diff.

Describe the solution you'd like
diff-*-theme.auto.conf support (or similar)

Describe alternatives you've considered
Using set-colors while in the diff but this does not work as it doesn't seem to be able to set diff specific keys

[kovidgoyal]

Do you really leave the diff kitten running over extended periods of
time that you need it to respond to OS color changes? I can see a
usecase for having the diff kitten auto select a color them on startup,
but I dont see much point in having it respond to color changes.

[ETLaurent]

It actually makes a lot of sense, very useful when manually changing the OS to light/dark mode.

For instance, when going outside 🌞 while we're in the middle of a huge diff, we'd change the OS to light mode and want the diff theme to automatically switch (without having to re-open it).

Anyway, thanks for implementing this because it works well (and is very useful)

dark	light