[vim] Set termwinkey to allow CTRL-W

Fix https://github.com/junegunn/fzf.vim/issues/1176

plugin/fzf.vim

@@ -836,6 +836,7 @@ function! s:execute_term(dict, command, temps) abort
         let term_opts.curwin = 1
       endif
       let fzf.buf = term_start([&shell, &shellcmdflag, command], term_opts)
+      call setbufvar(fzf.buf, '&termwinkey', '<c-z>')
       if is_popup && exists('#TerminalWinOpen')
         doautocmd <nomodeline> TerminalWinOpen
       endif