* feat: allow passing a custom function as a window picker
WIP
* fix: move logic expression to if statement
If `M.window_picker.custom_function()` returns `nil` then `pick_win_id()`
will run (the or part). We don't want that. More verbose, but better.
* feat(open): add window_picker.picker
* feat(open): add window_picker.picker
* style nit
* feat(open): add window_picker.picker
* docs: add window_picker.picker documentation
* docs: add window_picker.picker documentation
Co-authored-by: Alexander Courtis <alex@courtis.org>