fix(dotfiles): update Hammerspoon screen resolutions
fix(dotfiles): update Hammerspoon screen resolutions

Different laptop and external display at play now. Conveniently, both
the mid-2015 15" MacBook Pro and 2020 13" MacBook Pro (M1) both report
their resolution as 1440x900.

Also, my external resolution has changed.

Added `externalDisplay()` for symmetry, although I confess to not using
it at this point.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent