fix: Fix non-functional vim plugin (#659)

The vim plugin uses --expect and expects the key used to accept the
selection on the first line of output. Commit bcee1f4 "feat!: do not
check for expect before printing the argument of accept… (#625)" broke
that by not outputting anything if the selection was made using the
default "enter" key. We can workaround that by explicitly adding
"enter" to the --expect argument, so that it once again shows up in the
output.

Fixes skim-rs/skim.vim#25

plugin/skim.vim

@@ -937,6 +937,7 @@ function! s:popup(opts) abort
 endfunction
 
 let s:default_action = {
+  \ 'enter': 'edit',
   \ 'ctrl-t': 'tab split',
   \ 'ctrl-x': 'split',
   \ 'ctrl-v': 'vsplit' }