VS Code: Support fenced cddl code block syntax highlighting

[letmaik]

Reference: https://github.com/mjbvz/vscode-fenced-code-block-grammar-injection-example

I tend to embed CDDL in markdown files, and while GitHub doesn't support it at the moment, I think it would be a good start if this extension adds support for it. Then eventually it could be added to https://github.com/github/linguist.