Highlight code snippets in doc comments

[Centril]

Say we write something like:

/// Yada yada...
/// ```rust
/// assert_eq!(foo(), some_value);
/// ```
fn foo() { ... }

Unfortunately; the snippet inside the doc comment is not highlighted in VSCode.
It would be awesome if it were.

[Xanewok]

cc microsoft/vscode#20488 for embedding language grammars in VSCode