Add support for inline images in Markdown

[rock3r]

Inline images in Markdown are not trivial. They require using InlineTextContent, but that in order is not easy. In particular, since multiple images can have the same URL in an AnnotatedString, we need a way to keep track of, and generate, unique IDs for each image in DefaultInlineMarkdownRenderer. This also needs to work with extensions, which in turn will end up finding images and will need univocity of their IDs.

Unfortunately, IDs are plain strings, which is a giant pain in the arse.

An idea is to structure the IDs to contain a prefix saying "this is an image", then the image URL, and then an index, all separated by a newline or another non-URL-safe character