Added: Terminal CSI reporting of window and cell pixel size

Implement the following CSI escape sequences from
https://invisible-island.net/xterm/ctlseqs/ctlseqs.html:

> CSI Ps ; Ps ; Ps t
> [..]
>    Ps = 1 4  ⇒  Report xterm text area size in pixels.
>    Result is CSI  4 ;  height ;  width t
> [..]
>    Ps = 1 6  ⇒  Report xterm character cell size in pixels.
>    Result is CSI  6 ;  height ;  width t

Extracted from changes in https://github.com/termux/termux-app/pull/2973
by @MatanZ and adopted to play well with the just merged #3098 (.ws_xpixel
and .ws_ypixel values in winsize).