RFC: Add fern#smart#ready() to support await like mapping call

[lambdalisue]

To solve #107, fern#smart#ready() function has added. The function invoke a given expression once a fern buffer become ready thus users do not need to care the asynchronous calls.

To execute tcd:root action every after enter/leave action, first define <Plug>(my-fern-tcd-on-ready) action like

nnoremap <buffer><silent> <Plug>(my-fern-tcd-on-ready)
      \ :<C-u>call fern#smart#ready("\<Plug>(fern-action-tcd:root)")<CR>

Then, create <Plug>(my-fern-enter-or-tcd)/<Plug>(my-fern-leave-or-tcd) like

nmap <buffer> <Plug>(my-fern-enter-or-tcd)
      \ <Plug>(fern-action-enter)<Plug>(my-fern-tcd-on-ready)
nmap <buffer> <Plug>(my-fern-leave-or-tcd)
      \ <Plug>(fern-action-leave)<Plug>(my-fern-tcd-on-ready)

Top-Left indicates current working directory. The cwd changes when the root directory has changed.

More complex mapping example for change behavior on drawer/window

function! s:fern_init() abort
  nnoremap <buffer><silent>
       \ <Plug>(fern-my-tcd:root-ready)
       \ :<C-u>call fern#smart#ready("\<Plug>(fern-action-tcd:root)")<CR>
  nmap <buffer>
        \ <Plug>(fern-my-enter-and-tcd)
        \ <Plug>(fern-action-enter)<Plug>(fern-my-tcd:root-ready)
  nmap <buffer>
        \ <Plug>(fern-my-leave-and-tcd)
        \ <Plug>(fern-action-leave)<Plug>(fern-my-tcd:root-ready)

  nmap <buffer><expr>
        \ <Plug>(fern-my-open-or-enter-and-tcd)
        \ fern#smart#leaf(
        \   "\<Plug>(fern-action-open)",
        \   "\<Plug>(fern-my-enter-and-tcd)",
        \ )

  nmap <buffer><expr>
        \ <Plug>(fern-my-open-or-enter)
        \ fern#smart#drawer(
        \   "\<Plug>(fern-my-open-or-enter-and-tcd)",
        \   "\<Plug>(fern-open-or-enter)",
        \ )
  nmap <buffer><expr>
        \ <Plug>(fern-my-leave)
        \ fern#smart#drawer(
        \   "\<Plug>(fern-my-leave-and-tcd)",
        \   "\<Plug>(fern-action-leave)",
        \ )
  nmap <buffer><nowait> <Return> <Plug>(fern-my-open-or-enter)
  nmap <buffer><nowait> <Backspace> <Plug>(fern-my-leave)
  nmap <buffer><nowait> <C-m> <Plug>(fern-my-open-or-enter)
  nmap <buffer><nowait> <C-h> <Plug>(fern-my-leave)
endfunction

[lambdalisue]

It's a bit tricky. I'd like to find a better way.

[lambdalisue]

In favor of #112. Close this PR.