Method is_image

Returns `true` if link is an image link ```rust use orgize::{Org, ast::Link}; let link = Org::parse("[[https://google.com]]").first_node:: ().unwrap(); assert!(!link.is_image()); let link = Org::parse("[[file:/home/dominik/images/jupiter.jpg]]").first_node:: ().unwrap(); assert!(link.is_image()); ```

(&self)