Problem: canola had no way to open a browser in a normal split window;
only floating windows were supported via `open_float`/`toggle_float`.
`M.close` also crashed with E444 when called from the last window.
Solution: port stevearc/oil.nvim#728 — add `open_split(dir, opts, cb)`
and `toggle_split(dir, opts, cb)` mirroring the float API. Use
`is_canola_win`/`canola_original_win` window vars (not the upstream
`is_oil_win` names). Wrap `nvim_win_close` in `pcall` with `enew()`
fallback to handle the last-window E444 case.
Based on: stevearc/oil.nvim#728
Barrett Ruth
943bf750cc
