Soft kill terminal buffer

- Category
- [x] Question
- OS
- [x] Windows
- Vim
- [x] Vim

_fzf 0.17.1, lastest fzf.vim, gvim8 1-1358, windows 10, +terminal_
**Video [demo](https://drive.google.com/file/d/19bAI1tERLRPGP2TFA48ccb4wZmYogpXz/view?usp=sharing).**
Minimal vimrc:
```
command! -bang -nargs=* Find
\ call fzf#vim#grep(
\ 'rg --column --line-number --no-heading --color=always '.shellescape(), 1,
\ 0 ? fzf#vim#with_preview('up:60%')
\ : fzf#vim#with_preview('right:50%:hidden', '?'),
\ 0)

nnoremap :bdelete
nnoremap \ :Find
```
Produce steps:
- Issue this command in cmd under home directory (i.e: C:\Users\USERNAME\):
```gvim.exe --cmd "set nocp | set rtp+=~/vimfiles/pack/plugins/start/fzf | runtime! plugin/fzf.vim"```
- Type `:FZF`, try to kill terminal buffer with `` --> reuslt buffer killed.
- Open gvim, open an arbitrary file, try to find a pattern with `:Find` command (i.e `:Find abc`). After type `:Find abc` command, try to kill terminal buffer --> result buffer can't be killed.

So the expected result is that terminal buffer can be killed using ``