for image-left/image-right layouts add a setting to set image width

[alex-176]

for layouts image-left/image-right it'd be great to have a simple way to change how much space the image takes on the slide. currently, it's 50% and I did not find a simple way to change it.

e.g. in frontmatter:
imageWidth: 30% # to make an image take only 30% of the full slide width

maybe there is a workaround with <style> (that I did not find) but the straightforward setting is clearer.