Problem: nvim_win_get_config() does not return a window's "style".
Solution: always include it, and document `style=""`.
Always included so it can be used reciprocally with nvim_open_win() or
nvim_win_set_config(). (otherwise the config of a window with kWinStyleUnused
will not unset the kWinStyleMinimal style of another window if passed to
nvim_win_set_config, for example)
1901832f26
