Dynamic cast area target

[YaLTeR]

Something like niri msg action pick-dynamic-cast-area that lets you draw a rectangle on an output to cast just that rectangle.

Though, need to consider that we will want a dynamic cast workspace target too in the future, and area selection makes sense for it too, in addition to a whole output.

[YaLTeR]

Now that I think about it, arguably area makes more sense for a workspace than it does for output?