Add image Shortcode

[LukasGrebe]

Hi, i noticed package.json lists "@11ty/eleventy-img": "^0.8.2" as a dependancy. I've added the shortcode as per eleventy docs.

I've also modified the the shortcodes to be asnyc

[LukasGrebe]

hold on - I just noticed moving all shortcakes to async does not seem to work in some cases.

I'm not sure how to reproduce it (yet) but it appears that using the icon shortcode in nested njk templates leads to an error

[maxboeck]

this is a good addition, thank you!

regarding the async issue, I would opt to register just the image shortcode as async and leave the icon shortcode as-is, since it doesn't return a promise. may have to rethink that "register all shortcodes in a loop" approach in the eleventy.js config though...