feat: add ability to have scrollback window and paste window open at the same time

[mikesmithgh]

Currently, when you navigate away from the paste window it autocloses. It would be nice to have the ability to navigate back and forth while it is open or have the paste window in a normal window instead of a floating window. The idea is to allow you to view/select text from the scrollback buffer but keep the paste window visible for edits.

See #253 (comment)

[IndianBoy42]

btw, recently nvim_open_win works mostly uniformly for floating windows as well as splits (used to be only for opening floats) so making the paste_window configurable between float and split should be simple.

I think visually it looks like a split already so making it an actual split by default makes sense to me