[Markdown] Adding support for GitHub themed images

[lostintangent]

GitHub recently added support for appending #gh-dark-mode-only / #gh-light-mode-only to the end of a Markdown image's URL, in order to hide/show images based on the end-user's active color theme (Details). This PR simply adds the equivalent support to the VS Code markdown preview.

Note: I added this support to the built-in github extension, since the URL marker is technically GitHub specific, and therefore, felt appropriate being a part of the core GitHub capabilities. That said, let me know if this seems like it should be part of a different built-in extension 👍🏼

[mjbvz]

Should this be in https://marketplace.visualstudio.com/items?itemName=bierner.markdown-preview-github-styles instead?

[lostintangent]

The main reason I was thinking it might make sense to include this in a VS Code built-in extension, is so it would be available in Remote Repositories, GitHub.dev, Codespaces, etc. without needing to install anything else. That way it felt like a native experience for GitHub contexts, just like this is natively supported in GitHub.com.

That said, we could also just auto-install the extension you mentioned in GitHub.dev and Codespaces, and achieve the same effect. So I'm cool with whichever approach you think makes the most sense 👍

[mjbvz]

Thanks for the details @lostintangent

I agree having it in core makes sense then