Threads through a GuiPosition from mux window creation to allow it to be
used when the corresponding gui window is created.
SpawnCommand now has an optional position field to use for that purpose.
```lua
wezterm.mux.spawn_window {
position = {
x = 10,
y = 300,
-- Optional origin to use for x and y.
-- Possible values:
-- * "ScreenCoordinateSystem" (this is the default)
-- * "MainScreen" (the primary or main screen)
-- * "ActiveScreen" (whichever screen hosts the active/focused window)
-- * {Named="HDMI-1"} - uses a screen by name. See wezterm.gui.screens()
-- origin = "ScreenCoordinateSystem"
},
}
```
refs: https://github.com/wez/wezterm/issues/2976