Add support for kitty's automatic theme switching based on OS
color scheme. This creates the required auto theme config files:
- light-theme.auto.conf
- dark-theme.auto.conf
- no-preference-theme.auto.conf
Closes: nix-community/home-manager#6869
This allows better support for using kitty's built-in shell integration
method by setting `shellIntegration.mode' to `null', which makes it not
be set at all in the config file and disables modification to shell
initialization scripts by default. If this is used, the user should set
`settings.shell_integration' instead (if shell integration is desired),
which receives no special handling.
