Syntax highlighting in markdown code blocks

[JohannesRudolph]

I'm not sure how other languages do it, but it appears there's a way that vscode-language-dhall needs to register for supporting dhall code in markdown fenced code blocks: https://github.com/mjbvz/vscode-fenced-code-block-grammar-injection-example

As an example I'd like to have syntax highlighting in the following code block ( \ used to escape backticks):

\```dhall
let Monoid
    : ∀(m : Type) → Type
    = λ(m : Type) → List m → m

in  Monoid
\```

Right now there's no syntax highlighting in fenced code blocks.