fix: highlighting is lost when colorscheme is changed

New vim restores default highlight links, but fails to do it when there is no default link in the first place.

Without this fix, vim-exchange looses highlighting after `:colorscheme ...`

Relates to vim/vim#6970

plugin/exchange.vim

@@ -325,6 +325,7 @@ function! s:get_setting(setting, default)
 endfunction
 
 highlight default link ExchangeRegion IncSearch
+execute 'highlight default link _exchange_region' (s:enable_highlighting ? 'ExchangeRegion' : 'None')
 
 nnoremap <silent> <expr> <Plug>(Exchange) ':<C-u>set operatorfunc=<SID>exchange_set<CR>'.(v:count1 == 1 ? '' : v:count1).'g@'
 vnoremap <silent> <Plug>(Exchange) :<C-u>call <SID>exchange_set(visualmode(), 1)<CR>