Code block syntax highlighting

[dylans]

With #1055 I've re-enabled syntax highlighting for code-block. There are some things that should still be done after this lands:

• Get it working in the playground (CSS isn't loading correctly)
• Option to use the code-block plugin without syntax highlighting (likely via a configuration option)
• Option to show line numbers
• Option to disable wrapping
• Determine why Scala is throwing an error
• Styling for the dropdown list to match Plate
• Add tests when a language gets selected?
• Other options to choose language (e.g. hotkeys like triple-backtick+js)

[zbeyens]

Closing this as most items are completed with Plate v22.