image validation

[mortendk]

if a user adds "a false image" external of internal in the markdown file the build fails :/
the samme happens if somebody deletes the image

usecase:
Somebody takes a screenshot, paste to a word document saves as a doc it and wanna use it as an image
image: foobar.doc

Somebody removed the image foo and now stuff dont work

solution:
Test for filetype or test if if 11ty image can escape if the file isnt a png, gif, jpg, svg etc