✨ feat: add dimmable-image shortcode for dark theme

Reduces contrast and brightness of images when the dark theme is active.
Useful when invertable-image is not appropriate (photographs, paintings,
colour drawings…)

templates/shortcodes/dimmable_image.html

@@ -0,0 +1 @@
+<img class="dimmable-image" {% if src %}src={{ src | safe | replace(from="$BASE_URL", to=config.base_url) }} {% endif %} {% if alt %}alt="{{ alt }}"{% endif %}/>