Mark documentation comment blocks as Markdown.

[AndreasBackx]

This adds support for marking the content inside of a doc comment as markdown. Additionally the following identifiers are marked as documentation blocks as well:

• //!
• /*!

These were previously identified as the double slash / non-doc variant. These are defined in the Rust spec: https://doc.rust-lang.org/reference/comments.html

Examples of how this looks in VS Code:

It still defaults to the comment foreground colour for codeblocks in VS Code, though my guess is that this is a VS Code issue? Might even be an issue of my own theme.

Looking at the default theme, it definitely seems like an issue for my theme:

This comes from my original issue https://github.com/microsoft/vscode/issues/169676 and I hope to sync this to the PR for VS Code as well: https://github.com/microsoft/vscode/pull/169956. #20 also suggested this feature.

Let me know what you think. 😃