[FR] Add check for attachments in org-lint

[Gustavo Barros]

Hi All,

Org 'attachment:' links are essentially file links to local files and, 
while 'org-lint' checks 'file:' links for the existence of the 
corresponding files with `org-lint-link-to-local-file', as far as I can 
tell, the same check is not done for 'attachment:' links.

So, I'd like to kindly suggest that either "attachment" be included as a 
case in `org-lint-link-to-local-file' or that an equivalent check for 
attachments be included in `org-lint'.  I'd be much thankful.

Best,
Gustavo.