csi: implement private mode 2031 (dark/light mode detection)

* Recognize 'CSI ? 996 n', and respond with
  - 'CSI ? 997 ; 1 n' if the primary theme is active
  - 'CSI ? 997 ; 2 n' if the alternative theme is actice
* Implement private mode 2031, where changing the color
  theme (currently only possible via key bindings) causes the terminal
  to send the same CSI sequences as above.

In this context, foot's primary theme is considered dark, and the
alternative theme light (since the default theme is dark).

Closes #2025
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Daniel Eklöf 2025-04-20 12:48:37 +02:00
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@ -71,6 +71,11 @@
`key-bindings.color-theme-toggle` key bindings. These can be used to
switch between the primary and alternative color themes. They are
not bound by default.
* Support for private mode 2031 - [_Dark and Light Mode
Detection_](https://contour-terminal.org/vt-extensions/color-palette-update-notifications/)
([#2025][2025])
[2025]: https://codeberg.org/dnkl/foot/issues/2025
### Changed