Currently implemented on X11 only, this function returns information
about the geometry of the screen(s).
This is taken from the same source of information we use for the
`--position` CLI argument to `wezterm start`.
```
> wezterm.window.screens()
{
"by_name": {
"DisplayPort-1": {
"height": 2160,
"name": "DisplayPort-1",
"width": 3840,
"x": 0,
"y": 0,
},
},
"main": {
"height": 2160,
"name": "DisplayPort-1",
"width": 3840,
"x": 0,
"y": 0,
},
"origin_x": 0,
"origin_y": 0,
"virtual_height": 2160,
"virtual_width": 3840,
}
```
