Implement non-function type alias support in syntax highlighters

[leafpetersen]

This issue tracks support of the non-function type alias feature. See the enclosing meta-issue for details.

• Github syntax highlighting.
  • Github grammar for use with VS Code.
• highlight.js support (api.dart.dev, api.pub.dev, Dartdoc):
• CodeMirror support (DartPad).

cc @devoncarew @srawlins @mit-mit @keertip @franklinyow

[srawlins]

Looks fine in CodeMirror

[srawlins]

Also looks fine in highlight.js

[devoncarew]

VS Code / the textmate grammar looks good as well.