Relative images containing `:` are not handled correctly

[mhaack]

Description
If a BOYM content source uses relative image src paths containing a : the image is not processed correctly. Example /adobe/assets/urn:aaid:aem:75c50af1-19da-42fc-a7bd-57aecaa6af3c

helix-html2md/src/mdast-process-images.js

Line 42 in 225cb47

if (url.indexOf(':') < 0) {

> this check is not optimal to detect relativ images. An url.startsWith('/') would be a better check.