[Extension type] Syntax highlighters implementation

[itsjustkevin]

Cobbling together the list from past issues:

• https://github.com/dart-lang/dart-syntax-highlight/ (@DanTup)
  • Add support for inline classes dart-syntax-highlight#41
  • Change "inline class" to "extension type" dart-syntax-highlight#50
• https://github.com/highlightjs/highlight.js (@srawlins)
• https://github.com/codemirror/CodeMirror/ (@domesticmouse)

[eernstg]

Please note that this feature has been renamed: It is now the 'inline class' feature.

[DanTup]

@itsjustkevin do you have a script to create issues like this one? I was adding some items to the server list and wasn't sure whether the syntax highlighting (that doesn't come from the server) is recorded anywhere?