[Feature Request] Support for OSC 11 (text background color)

[Ziyann]

OSC 11 is used by some tools to adjust their color scheme, with Vim being one of the most notable examples.

The following sequence should return the current background color: echo -e '\033]11;?\007'

Example on Tabby:

$ echo -e '\033]11;?\007'

$ 11;rgb:0000/0000/0000

$ vim -c "set background?"
background=dark

Example on KiTTY:

$ echo -e '\033]11;?\007'

$ 

$ vim -c "set background?"
background=light

http://www.talisman.org/~erlkonig/documents/xterm-color-queries/ https://github.com/vim/vim/blob/master/src/term.c#L5685-L5697