Add set_background_color on TuiBackend

[alexanderbh]

If this is not something Anathema wants to support maybe a different change could be to expose Screen/Output from the backend to the user. Then this could be implemented in the user app instead.

OSC 11 is not supported everywhere. But it is in most of the terminals that people use. It is not a standard but more of a defacto standard.

I have tested with success in: Windows Terminal, Alacritty (windows), Ghostty (mac), Terminal.app (mac), iTerm2 (mac).

Run the example

cargo run --features osc --example terminal_background