fix: winbar not being included in window height calculations

[0xAdk]

Fixes a few off by one rendering errors such as #18 and an unreported issue where when scrolling with the mouse the scroll bar is able to move off the screen by one

this utilizes the change in nightly https://github.com/neovim/neovim/issues/14090#issuecomment-1225699877

I am torn on if using the non api function or creating a new winbar aware utility function that utilizes the api is a better idea. I don't know how obscure the difference between winheight() and nvim_win_get_height() is. It might lead to confusion while reading the code :/