Add JS Syntax Highlighting to Markdown Snippets

[BrooklinJazz]

Should be a simple fix to include js in the list of allowed languages. See https://github.com/livebook-dev/livebook/pull/1246 For an example.

I'll happily push up a PR to fix this once I have some time later today if no one else grabs it.

[jonatanklosko]

Sounds good to me!