Allow multiple drawers with horizontal split (`<C-w>s`) on drawer style viewer

```vim
#!/usr/bin/env -S vi -u
" $ uname -r
" 5.19.7-arch1-1
" $ has
" ✓ vi 0.8.0
" $ cat test.vim
set runtimepath=$VIMRUNTIME
set runtimepath+=~/.local/share/nvim/repos/github.com/lambdalisue/fern.vim
" $ chmod +x test.vim
" $ ./test.vim
```

1. `:Fern -drawer ~<CR>`
2. `<C-W>s`
3. `z`

Nothing happens.

Expect behaviour:

```
Width ratio [2 -> 1-10]:
```

Thanks.