vim:keys:<leader>": Convert only pairs of quotes

When converting double to single quotes with `<leader>"`, only pairs of
quotes should be converted. This lowers the false positive rate by
trying to make sure that the double quotes are really used for string
quotation and not for something else (e.g. exactly these mappings).

.config/vim/vimrc.d/40-keys.vim

@@ -244,9 +244,10 @@ noremap ][ ]]
 nnoremap <leader><space> :silent! %s/\v\s+$//<CR>
 vnoremap <leader><space> :<C-u>silent! '<,'>s/\v\s+$//<CR>
 
-" Convert double quotes to single
-nnoremap <leader>" :silent! %s/"/'/g<CR>
-vnoremap <leader>" :<C-u>silent! '<,'>s/"/'/g<CR>
+" Convert double quotes to single. Convert only pairs to lower the false
+" positive rate.
+nnoremap <leader>" :silent! %s/\v"([^"]*)"/'\1'/g<CR>
+vnoremap <leader>" :<C-u>silent! '<,'>s/\v"([^"]*)"/'\1'/g<CR>
 
 " Keep selection when changing the indentation in visual mode
 vnoremap > >gv