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

[Freed-Wu]

#!/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.

[lambdalisue]

Fern currently does not support multiple drawer window on same tab page and that cauze this issue because zoom action is only available on drawer window.

However, it's quite difficult to define "what is the correct behavior" for multiple drawer window so it's likely a limitation.