src: Add a `window_range` default expansion
src: Add a `window_range` default expansion

This commit adds a `window_range` default expansion that holds the
coordinates and size of the buffer-space on the window.

Fixes #675

Somasis/kakoune,lenormf/kakoune,jkonecny12/kakoune,danr/kakoune,mawww/kakoune,casimir/kakoune,casimir/kakoune,danr/kakoune,Somasis/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,lenormf/kakoune,lenormf/kakoune,danr/kakoune,danr/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,occivink/kakoune,Somasis/kakoune,alexherbo2/kakoune,casimir/kakoune,mawww/kakoune,mawww/kakoune,occivink/kakoune,alexherbo2/kakoune,occivink/kakoune,mawww/kakoune,occivink/kakoune,casimir/kakoune,Somasis/kakoune,jkonecny12/kakoune,lenormf/kakoune