Allow adding light/dark images in READMEs that follow browser theme#1106

<picture> <source media="(prefers-color-scheme: dark)" srcset="https://user-images.githubusercontent.com/25423296/163456776-7f95b81a-f1ed-45f7-b7ab-8fa810d529fa.png"> <source media="(prefers-color-scheme: light)" srcset="https://user-images.githubusercontent.com/25423296/163456779-a8556205-d0a5-45e2-ac17-42d089e3c3f8.png"> <img alt="Shows an illustrated sun in light mode and a moon with stars in dark mode." src="https://user-images.githubusercontent.com/25423296/163456779-a8556205-d0a5-45e2-ac17-42d089e3c3f8.png"> </picture> 

Add ability to add light/dark images in READMEs, e.g [using GitHub format](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#specifying-the-theme-an-image-is-shown-to), so the image could adapt to current browser theme ```md <picture> <source media="(prefers-color-scheme: dark)" srcset="https://user-images.githubusercontent.com/25423296/163456776-7f95b81a-f1ed-45f7-b7ab-8fa810d529fa.png"> <source media="(prefers-color-scheme: light)" srcset="https://user-images.githubusercontent.com/25423296/163456779-a8556205-d0a5-45e2-ac17-42d089e3c3f8.png"> <img alt="Shows an illustrated sun in light mode and a moon with stars in dark mode." src="https://user-images.githubusercontent.com/25423296/163456779-a8556205-d0a5-45e2-ac17-42d089e3c3f8.png"> </picture> ``` Example of how it looks ^{[source](https://www.stefanjudis.com/notes/how-to-define-dark-light-mode-images-in-github-markdown/)}